I am a holy priest and I love it. I don't run around looking for PvP because my opinion of PvP right now is that I am less than impressed. When I do PvP I have a warrior and rogue to rip shit up. I'm much more group oriented. Don't get me wrong I loved my shadow spec, but I enjoy holy also. That said, you don't really NEED to have holy for anything. Now this is coming from an alliance perspective, but we usually have so many paladins second and third healing MT's in MC that it's more about mana consumption than anything. Of the priests that we have for MC, 5 are shadow/disc, 1 is disc/shadow, and I'm holy/disc. However, the tanks would rather have me as their main healer over the shadow priests. I took the 5 points of master healer and honestly thought I had wasted 5 points that could have gone into meditation (with my regen equipment I have 30 mana/5s + 300 spirit) up until MC, but I find I can use greater heals in places where others can't. I just feel more comfortable with it.
Now, I did have a holy/shadow build for a long time and I will say that you will be shooting yourself in the foot. If you plan on soloing, just stick with the shadow/disc build until 58-59 and then switch if you want to. Without the high end shadow talents you're still going to kill shit very slowly.
Gibe is right though, the discipline talents are probably the most important. That's why it's not very efficient to go tri-tree, but I don't think that it's a terrible idea. Mental strength is a must if you're going to put points into the tree. Inner focus is nice, but I use it same as above... pofII and when oom. Meditation I'm on the border. I think it becomes more attractive if you have a high sprit and +mana-regen build. Either way, with 8.5k mana or 6.5k mana, you're going to run out of mana at some point in those long fights and discipline will help you get the most out of your mana.
I think the best chance for a tri-build would be:
Saved Template
Class: Priest
Level: 60
Discipline Talents (20 points)
# Unbreakable Will - 5/5 points
Increases your chance to resist Stun, Fear, and Silence effects by 15%.
# Improved Power Word: Shield - 3/3 points
Reduces the duration of your Power Word: Shield's Weakened Soul effect by 15 seconds.
# Improved Power Word: Fortitude - 2/2 points
Increases the effect of your Power Word: Fortitude spell by 30%.
# Mental Agility - 5/5 points
Reduces the mana cost of your instant cast spells by 10%.
# Mental Strength - 5/5 points
Increases your maximum Mana by 10%.
Holy Talents (10 points)
# Improved Renew - 5/5 points
Increases the amount healed by your Renew spell by 15%.
# Spiritual Healing - 5/5 points
Increases the amount healed by your healing spells by 10%.
Shadow Talents (21 points)
# Blackout - 5/5 points
Gives your Shadow damage spells a 10% chance to stun the target for 3 seconds.
# Improved Shadow Word: Pain - 2/2 points
Increases the duration of your Shadow Word: Pain spell by 6 seconds.
# Shadow Focus - 3/5 points
Reduces your target's chance to resist your Shadow spells by 6%.
# Mind Flay - 1/1 point
Assault the target's mind with Shadow energy, causing 75 damage over 3 seconds and slowing the target to 50% of their movement speed.
# Improved Psychic Scream - 2/2 points
Reduces the cooldown of your Psychic Scream ability by 4 seconds.
# Improved Mind Blast - 5/5 points
Reduces the cooldown of your Mind Blast spell by 2.5 seconds.
# Improved Fade - 2/2 points
Increases the duration of your Fade ability by 10 seconds.
# Silence - 1/1 point
Silence the target, preventing them from casting spells for 5 seconds.
This gets what I think are the best talents you can get mid range in the trees without too many useless talents.