First off, my objections have nothing to do with the "they aren't in the source literature" argument. My main beef with them, especially in OD&D, is that they are almost on par with fighters and they have spells. Not only do they have spells, they have access to every clerical spell out there, whether they are required to keep spellbooks or not. That makes them slightly above magic users, in that they can wear armor, use most weapons, and have access to their full spell list from 2nd level on. All of this is theirs for a seriously paltry XP table.
Now, I know that "access to their full spell list" may cause some of you to cite the brevity of said spell list. Accordingly, that may not seem such an advantage. I contend that it is a huge advantage due to additional spells. If additional spells, say, cherry-picked from Greyhawk, are introduced, the cleric has access to all of those as soon as they hit the street. Just adding one or two spells per level will increase the value of this advantage tremendously.
I have recently turned to Mr Moldvay's Basic for a possible solution to the Cleric Problem. According to a strict interpretation of those rules, with regard to magic users, a spell book may only contain as many spells as the magic user can cast in a day. This has the interesting side effect of making all magic users Specialists. I like this, very much. I think it can also be applied to clerics, and have several benefits, vis-a-vis my personal issues with the class.
- It promotes a sense of service to a particular patron without any heavy-handed rules for such
- It limits their spell abilities, which mitigates their low XP requirements
- It brings them more in-line with magic users and fighters, power-wise
I think this also feathers in nicely with the notion that clerical spells are divine. Done this way, the power that makes the spells work could be seem as coming from the cleric's zeal. I've always had problems with players of clerics doing things with their spells that their gods may not really approve of. Well, they can still do that using this idea, but at least the power for the spell is coming from within, rather than directly from the god.
Also, and this is a personal campaign-centric thing, I wouldn't allow clerics and paladins in the same campaign. Clerics are paladins, in that they are the righteous hand of their god. They are gifted with tremendous zeal coupled with a clarity of vision and willingness to battle forces opposed to their god. They are not kindly old men leading worship service on holy days, baptizing babies into the faith, and blessing crops every spring. They go into the dark places and meet evil head-on. Unless, of course, they've turned to evil and are hell-bent to bring suffering to all those who oppose them.