With the combined squishy-factor of both classes, a rogue/priest is going to seem very vulnerable in the low levels, but actually it shines as a PvP combo from the low- to mid-levels. The first three rogue elite skills combined with a self-heal are very favorable to killing fast enough to more than counteract having low physical defense and health. The level 15 elite skill Quickness Aura immediately boosts your dodge rating, and the next two elites boost DPS. Combined with the standard high-dexterity gear, and sporting dual daggers, this build can quickly shred through opposition.
Many players agree to forgo pumping TP into the rogue's Poison as you'll become too strong to really benefit from it. I'd also recommend keeping Dagger Mastery, Wound Attack, Shadowstab, and Wound Tear raised high to maximize DPS during low- to mid-levels. I'd also sacrifice some TP -- if necessary -- to Quickness Aura. Make sure to take Off-hand Training at level 16, then continue to play around. The great thing about all RoM classes is that each skill has a very wide range of possible levels, making any written-in-stone skill-builds possibly harder to determine than in other MMOs.
I especially love the uniqueness of taking the priest as the primary class in this combo, in the low levels. Bone Chill becomes available at level 8, Snake Curse at 15, Chain of Light at 18, and Infectious Wound at 20. All of these are ranged, and while Infectious Wound is direct damage, the other three are damage over time. This build is especially deadly against melee characters who have to get in close to start dealing damage. Chain of Light is a six-second DoT that is breakable, so make sure to spend TP on Wave Armor. Wave Armor will soak up that damage, allowing Chain of Light to stay on the target. Keep pumping TP into all of the above, and spare some for Cleanse and Regenerate as the character's level and TP increase. I like Regenerate better than Heal and Urgent Heal, but all heals will definitely have their uses.
I think it's debatable as to how many levels the PvP builds can go before they start to lose effectiveness. At higher levels, it's difficult to keep more than three or four skills raised to match character level. Since everyone gets a free TP reset stone at level 30, it's a good place to start thinking -- and working on -- a PvE support-build.
As we look ahead to level 50 and beyond, there are some interesting choices for group support. A build with priest as the primary class could go all-out healer but could also be an extremely fun DPS priest. Condensing heals to Group Heal will free up plenty of TP for Snake Curse and Bone Chill. The trickle of TP that only allows for three or four skills to stay maxed -- as a character gets closer to level 50 -- will almost have to be sacrificed to raise a wider variety of skills that won't be quite as effective at high level. It's not a bad idea, and adding in Magic Barrier, Shadow Fury, and Amplified attack will boost everyone's attack power while adding magical defense. It's a matter of TP, though, and a character still on the way to level 50 may do best to concentrate on the support skills I mentioned. Beyond 50, and as level cap draws closer, more TP will become available, allowing us to start adding all these other skills.
It's important to note that a character can be "effective" without the need to keep any skills maxed out, and some TP can be spent in quite a few skills up to a certain level. Of course, I think that if you concentrate on a handful of skills (keeping them significantly higher than the rest and closer to the character's level), combat will be much more fruitful for you. But, any and all one-time-activate skills that don't require leveling should definitely be purchased.
Rogue/scout combos are currently pumping out more DPS compared to a rogue/priest, but that doesn't make rogue/priest obsolete. DPS is still very effective, and there's the addition of heals to consider, too. At higher level, but still below 50, more TP can be put into Informer for increased party crits, Quickness Aura to increase everyone's dodge rating, and Magic Barrier. As TP becomes more available, adding in Mana Surge will allow you to gain MP back through the use of Magic Barrier. I would slowly build support skills first and rogue-DPS skills second, since rogue/scouts will be more popular for just DPS.
Rogue/priest is a great class combination for those who want to build a low-level twink or level fast to endgame. It's a fun solo class, but it needs more help from groups at high level. A Fruit of Forgetting will allow you to replace one of the classes while keeping the other one high level, but high-level rogue/priest combos offer some unique and fun buffs for group play that make the combo worth keeping.
If I were to start adding gear modding to the equation, I think the universe would implode. It would be far too much to try to cover, but that's partly what makes RoM so great. All the different class combos and gear modifications really make every class useful in more than one way. It's more about finding the fun ways to use each class, and less about what ways need to be found before you can have fun. The rogue/priest -- and even the skill-builds -- I've mentioned, are just a few of the many ways to build a character that is still effective and fun to play. I'm sure veterans of RoM have found some great ways to build characters, but I'm more interested in finding out if anyone's made a character more unusual than any of the rogue/priest builds I've mentioned.