I do also think that they can be interesting as sort of combat medics, acting as a tougher support class to the front line fighters. Essentially second line "shaft of the spear" battle logistics. But maybe that's more apropos to a wargame than rpg/dungeon exploration.That right there is the crux of the issue for me. As far as I'm concerned, D&D is a wargame (I refer to [TO]SR D&D here and following). Storygamers sometimes say that derisively, but over the last couple of years I've come to terms with owning it.
And clerics are a damn handy unit.
Among wargames, D&D has some interesting properties. D&D is refereed; this is true of some (Stargrunt and Dirtside, for example, recommend a ref), but not most wargames. D&D is highly asymmetric, in terms of information asymmetry between referee forces and PC forces, force composition / capabilities (classed and leveled characters versus monster HD and special abilities, often numerical asymmetries), and structure of play (PCs typically proactive, on offense, and logistically-bound). D&D is typically played cooperatively, with a group of players and a referee, extending that asymmetry into the processing-power domain (one DM cannot out-think four typical players) but adding a group dynamic which must be carefully managed for optimal play (at the party-scale). Finally and obviously, D&D is campaign-focused to an extraordinary degree; the campaign rules, for adding units to your party and for units gaining new abilities, probably outweigh the combat rules (not that this stops anyone from running one-off battles / "one-shots").
The only wargame-qua-wargame that I know of with similar properties is Charlie Company, which I have not acquired. Space Hulk hits some of those properties (highly asymmetric, similar indoor environments, often played many-versus-one and in campaigns) but is pretty much never refereed, and their campaigns lack the continuity and advancement of D&D campaigns (due in part to tremendous casualty rates / poor human win ratio).
A couple of questions naturally follow. What are the consequences of viewing D&D as a wargame? Is D&D a good wargame? How do I run a sensible science-fiction campaign wargame along similar lines? I have partial answers to consequences, probably in a future post. I'm not sure what makes a good wargame, particularly for such a weird combination of attributes. And I really haven't thought about the third question yet.