I would say the following with melee and cleric comparison.
Buff up a level 20 cleric with his own spells, and with the other caster classes (Bard, druid, wizard) and feats.
Now buff up a level 20 fighter with the clerics and the other caster classes spells and feats.
Cleric will likely beat the Fighter in AB. No matter what you do. HE will have one less attack but that is the only difference. The divine power, favour added together will easily balance out the 4-5 ab difference. Also same with the ranger as well, and ab wise barbs have it a bit better.
Now rogue skill dumping: That and evasion. Evasion is worth a lot.
Cleric vs Meleer disjunction? Any of them gets disjunctioned it is over, without buffs each and everyone dies. Maybe the cleric can rebuff the fighter maybe he cant, maybe he can run away maybe he cannot. (Maybe if he buffs the fighter he can use him as a meatshield.) Also he can easily regain lost AB or AC with recasting the Divine shield and power. Whereas fighter can enter ICE +10 AC -10 AB...and likely will still be hit. (Just think on all those nasty +30 AB critters)
About soloer clerics:
-they exist
-most players support them because:
a, you ask him first to help you retrieve corpses, because he can, since he is a solo build, you will not get much rp(most of the time, but it is not necessary true!), but he will get the job done. And you want only THIS.
b, you buy items off from him, accept his gifts, venture with him to places even if you cant do there anything, why? Because: Free xp, free loot, free sightseeing. You will be his tourist, just he pays you to see how awesome he is
c, you do not stay persistant how you act towards him. if you have the chance to ask help from him, or from two lower level chars, you will ask him, even if you despise the character due to IC reasons or whatnot. OOC rescue before IC reasons and play.
The you in this context does not mean everyone, it means to those who act this way, you know who you are or you do not. Just as long as it is rewarding both
XP, Item, community and whatnot wise to do so, nothing will change.
//I also understand there are a number of clerics who can only buff folks from same religion or themselves or whatnot, so that is of course exluded//