Due to the uneven scaling of every ability (as no ability functions exactly the same as another), it's truly impossible to know what the best rotation for any given rogue may be. Each case must be examined individually to see if the stats are above or below certain 'decision thresholds'. This has been evident as we see Combat rogues nearing the armor penetration soft cap dropping Rupture from their rotation, as well as Mutilate rogues dropping Rupture to bring about higher Envenom buff uptime. The fact is that without the proper gear to support armor penetration, dropping Rupture from your rotation may very well be a DPS loss.
A rogue-playing friend of mine recently reactivated his account, and is sporting 4/5 Tier 8 with nearly no armor penetration on his gear and agility gems in every slot. He had been reading on Elitist Jerks that Ruptureless was the new favorite rotation and whether or not he should take his points out of Blood Spatter and Relentless Strikes. I worked with him to explain that due to his particular gear setup, agility and rupture were more valuable to him than a rogue in full ArP gear with an ArP proc trinket. What's good for one rogue may not be good for another, and so knowing which stats benefit you the most will ensure you make intelligent gear choices.
For rogues, haste is essentially a split between a white/poison damage boost and an energy regeneration bonus. Haste gives you more white swings, which in turn gives you more poison procs, making it very valuable for Mutilate's poison-centric damage model. It also yields more Focused Attacks and Combat Potency procs, making it the only stat to boost your energy production. Because both Combat and Mutilate value energy in different ways, there is again a difference in value for each spec.
It's not a bad stat for either, but Mutilate typically sees the greater benefit from energy-based attacks. On a recent Saurfang fight in ICC25, my Combat rogue had about 26% of his damage coming from energy-based attacks (Sinister Strike and Eviscerate), while a similarly-geared Mutilate rogue saw 30% of his damage from Mutilate mixed with Envenom, which did not include the bonus Instant / Deadly procs he saw from the Envenom buff. I won't shy away from haste, and I recently purchased the Herkuml War Token, but I also realize that haste is not my 'favorite' stat.
Armor Penetration can be a confusing stat, because its value rises as you stack more and more of it. The damage bonus you see when moving from 5% to 10% ArP is nowhere near the damage bonus you see when you move from 95% to 100%. Due to this scaling mechanic, you almost want to avoid Armor Penetration on your gear until you have a gear set which would bring you close to the soft cap (including an Armor Pen trinket), and then switch all of your gems at once.
In addition, Combat sees a much higher gain from Armor Penetration than Mutilate can expect. My Combat rogue deals about 70% physical damage, meaning 70% of everything I do is affected by Armor Penetration. A Mutilate rogue, on the other hand, will only see about 45% of their damage coming from melee attacks. Because Armor Penetration benefits a Combat rogue, they are also more likely to have a higher ArP value, further pushing ArP towards Combat and away from the Haste-loving Mutilate builds.
One trinket to rule them all:
Enter Deathbringer's Will, which is the very fun trinket that Saurfang25 drops if you can topple him. With 155 Armor Penetration on the trinket as a passive boost, Combat Rogues (along with Blood Death Knights, Warriors, Feral Druids, and Marksman Hunters) will be salivating immediately. Every once in a while you'll have a brain-dead Ret paladin (is that redundant?) try rolling on it, which should be quickly squashed by the true physical DPS classes quickly.
Clearly it benefits a Combat rogue's 70% physical damage more than a Mutilate rogue who would see it affect less than half his damage, but it even gets worse. The random procs are different based on the class you're playing, and for Rogues, ArP is one of the possible options. An ArP passive with a possible ArP proc is not very valuable to a Mutilate rogue at all, while it's Combat rogue Heaven. The other two rogue procs, AP and Agility, are fairly vanilla and are great for both specs (as long as the Mutilate rogue is avoiding the crit cap).
It was discovered that Death Knights actually receive different procs based on their spec. A Blood DK, which does largely physical damage, scores ArP from the proc. An Unholy DK, who's damage is mostly magical, gains the better stat for Unholy: Haste. Why doesn't it work this way for Mutilate? Haste for Mutilate and Armor Penetration for Combat would be the best of both worlds, even if the passive boost is slightly lackluster for a Mutilate build. This idea was brought up on the Damage Dealing role forums, and apparently is actually going to be looked at and possible revised!
Knowing which stats are valuable to your class AND spec, and knowing how to balance those stats around caps and minimums becomes a large part of a raiding rogue's required knowledge. You're not alone in this, every rogue will struggle with this at some point in their lifetime. Consult a spreadsheet first for guidance, make decisions based on your gear on hand instead of what works for the best geared rogues in the game. Especially for the rogues who are just starting at 80: make sure you're looking at what's best for YOU instead of a general plan for every rogue. Knowing what you're doing at every gearing level will provide you with the best damage results you can achieve, which is like an upgrade in itself!