First of all, I'm a big supporter of non-standard specs. I think that your own play experience should help determine where you spend your talent points. However, I do think that you should at least examine why certain specs are tried and true, and how certain talents outperform others. If you never play in any type of group (instance, raid, battleground, arena, etc), then how you spec only affects you. Once you start spending time with other players, though, the way you play and spec starts to directly impact your teammates. Raids especially are all about teamwork, and if you're not contributing 100% in all the ways that you could, you start to become a liability. You become less of a liability in dungeons where fights are generally shorter, but you're still not living up to your potential.
I'm just going to jump right into it -- the first build I'm looking at today was the one sent in the email earlier this week.
The good: This build is a variant of an old school Hemo spec for PvP. Unfortunately it omits Improved Gouge from the Combat tree, and Ghostly Strike wasn't taken because "in a raid if you are dodging, something has gone wrong." It also opts to take Seal Fate instead of other talents, like Quick Recovery and Murder. The reader says that he created this build for other rogues in his guild so that points could be moved around for raiding, mentioning that many of the rogues in his guild are currently Mutilate spec and are having trouble keeping up in raids. It does pick up Preparation, Premeditation and Cold Blood, all of which have their uses in PvP.
The bad: It varies a bit from a more standard Hemo PvP spec (it actually reminds me of the pre-BC "Ganker's Delight" 21/8/22), but not in ways I would suggest. Not picking up Quick Recovery is a mistake, especially if you're just starting out in arenas. Without Improved Gouge this build also costs you some time on your stunlock. Using Seal Fate with this build is potentially OK for dungeons, but since more and more players are walking around with resilience these days I think it would limit its effectiveness in PvP. The biggest place this build would fall down is in raids -- most of the damage is of the burst variety, enhanced with talents like Cold Blood and Premeditation. Even with a large amount of melee DPS benefiting from the Hemo debuff, I couldn't see any Rogue using this build keeping up with damage in a raid, as there's just no talents to support sustained damage. Some talents, like Sleight of Hand, are wasted on PvE with a build like this -- you likely won't be causing enough threat to need to feint, so you don't need it to be 20% more effective.
The ugly: I would keep this build away from raids and heroic dungeons. You could use this as a PvP build, but the loss of Improved Gouge will effect your ability to stunlock. This would probably be best utilized by rogues who just look for people to gank out in the world, where the fight should be short and they can make the most use of their burst damage. If you want to raid or instance, check out a heavy combat build (see yesterday's Build Shop). If you want to PvP, consider one of the specs below.
Hemo Mace Build - 0/33/28
The good: Picks up Adrenaline Rush, Blade Flurry, Riposte, Hemo and Preparation. There are many variants of this basic build (including one that uses Sinister Strike instead of Hemo as the primary attack), but this is a solid build for PvP. It's a shame that Preparation no longer resets AR, but combo point generation is good and Riposte is quite strong again, now that there is no more immunity to disarm effects. You could also easily shift points around into Slice and Dice and do fine in PvE encounters as well. In larger raids with enough melee DPS, the hemo debuff adds a good amount of extra raid DPS at the expense of personal DPS for the rogue.
The bad: The big bummer here is that Prep no longer resets AR, which made this build perform extremely well in arenas. This build does lack many sustained damage talents that are useful in raids, but should be competitive in dungeons.
The ugly: While this build does well in PvP encounters, I wouldn't recommend taking this for raids if your guild is still running 10-mans, as the extra damage caused by the hemo debuff isn't worth the loss of the rogue's personal DPS.
Shadowstep Swords/Fists/Maces - 18/0/43
The good: Picks up most of the talents from the Assassination tree usually used in Combat builds. Also uses Hemo, so larger raids will benefit from the debuff. Performs quite well in PvP with the nice bag of tricks from the Sub tree, and Cheat Death adds to survivability. Can do competitive DPS in dungeons.
The bad: Lacks the added mobility of a 0/20/41 build that picks up Improved Sprint, which makes it more difficult to be kited. This spec also sacrifices all the sustained damage talents in the Combat tree -- not having Improved Slice and Dice will really hurt when trying to raid with this build. Using swords/maces/fists instead of daggers helps, but ultimately won't be able to keep up with PvE spec rogues, and will take an even bigger personal DPS loss in raids compared to the Hemo Mace build above.
The ugly: I have a Rogue in my guild running with almost this exact spec, and I'll tell you what I basically told him: it's a fine build for PvP and the occasional instance, but far less desirable for raids.
Combat Mutilate - 41/20/0
The good: Mutilate is fun to play, does strong damage against targets that are poisoned, and can put out some serious burst damage for PvP encounters. Great combo point generation means that this build is also capable of a long stunlock. Improved Kidney Shot can help group DPS against targets that can be stunned, and this build picks up enough Combat talents to help with sustained damage in a raid. So long as your targets can be poisoned, this build can really pump out some damage in a raid or dungeon.
The bad: Mutilate has the most demands placed on it for doing optimal damage -- you must have two daggers, you must be behind the target, and the target should be poisoned. In fights where you move around a lot or the target is immune to poison your DPS suffers greatly. Combo point generation is enhanced by crits, which in arenas means that you can expect to come up short on combo points quite often if your opponents have resilience. Having to attack from behind can be difficult in PvP on the best of days, and this problem can be compounded by a high latency.
The ugly: This was my build for a long time. I still find it very fun to play, and so long as your target can be poisoned you can stay competitive with damage if you keep your DPS cycles tight. However, I would stay away from this build for PvP -- latency issues, increased energy cost, and the poisoning requirement makes this build difficult. If you're looking to manage some cooldowns, have fun, do some nice DPS, and can deal with latency / positioning (for PvP), this might be the build for you.
This is really just a small sampling of the amount of builds out there. Some of the hybrid builds available can perform in several areas, as one would expect; however, for the Rogue who wants to raid seriously I would still suggest a Combat heavy build, as they are unmatched for inflicting sustained damage.
[Thanks for your email, Bryan!]