Just to provide a counter perspective to what Gilga said - I'm a hybrid spec druid healer (specced for pvp healing), I have 19 (or so) less pts in healing than gilga does. None of the 19 pts I have chosen outside of resto benefits my role as a raid healer at all. I seem to do fine in raiding. XD
Almost any talent tree has a point of diminishing returns. Once you hit that pt, you still gain *some* benefit from continuing to put pts into the tree with regards to your primary role, but there are also counter benefits at that point to diversifying your abilities because pt for pt overall (for solo, pvp, heroic, etc) other trees provide more value.
As an example for druids - There is benefit to putting the full 61 pts into resto. This is what i sacrifice: 20% threat reduction on heals, faster healing touch, more mana efficient healing touch and tranquility, stronger healing tough, un-interruptible tranquility, more spirit.
Now to talk about the diminishing returns on each:
20% threat reduction - probably the hardest to swallow losing. I am almost always the first healer dead in mordos raids. I keep up very nicely in TPS with some of our DPS. XD but on the other hand, very few boss fights do i worry about this. boss fights are generally what matter.
talents related to healing touch: I seriously cast healing touch without nature's swiftness maybe... 2 times per night. keeping that in mind, those would be 13 talent pts to modify a spell that I cast less than a dozen times in a night.
tranquility talents - I use tranquility maybe... 2-3 times a night, and with barkskin it's uninterruptible anyways.
moar spirit - spirit is quite nice for tree druids, but... not *that* nice.
Ok, now, what do I gain for saccing those 19 pts?
100% nature's grasp - key for getting rogues, wars, enhance shams, hunter and lock pets off you in pvp.
Un-interruptible by dmg roots and cyclone - uninterruptible CC? - yes please.
imp demo roar - this is probably the least worthwhile but i need it to get deeper in the tree - it still does reduce incoming melee dmg
imp bash - +2 sec on stun for pounce and bash? yes please.
Thick Hide - 10% more armor - not as useful in caster form, but it's multiplied in bear form - which is important for pvp again.
feral charge - druid intercept? yes please.
That's my thought process as I'm deciding my spec. By saccing 19 pts in spells that although they are part of my primary role, they are used infrequently, I'm able to gain significantly more survivability in pvp.