The argument for such a well-endowed class boils down to the fact that defensive strengths and versatility do not necessarily make for a fun or overly powerful character at the table.
If I burn lots of spells to prepare, I can be 110% as good as an unbuffed Fighter. (And smart Fighters can buff, too.)
If I burn all my top spell slots, I can be 90% as good as a Wizard for a few rounds.
It is simply not a given that either of the above will be practical for a PC in a particular adventure. The best DMs know how to keep the pace of combat in doubt, so neither of the above tactics are necessarily wise. What if you burn your best buffs and the high value targets retreat? What if they counterattack when your buffs have run out?
As Thurbane correctly points out, the overwhelming awesomeness of the Cleric or Druid only appears after a few rounds of buffing. Before that they are both unremarkable.
The deeper issue is that Resource Balance is less important than Dramatic Balance.
Most players do not care about what the sophisticated spreadsheet cooked up by some design wonk says about the normalized effective combat value of a particular build. What they care about is how often everyone at the table says: "Dude! That was a cool move!"
Players do not get frustrated because someone else's PC averages 11.2% more damage per round than their own PC. Players get frustrated or fail to have fun because they have too few special schticks that grab the spotlight when compared to other PCs.
Spreadsheets and average damage are about Resource Balance.
Dramatic Balance is about getting opportunities to wow other players with their schticks.
The Cleric and Druid look great on the spreadsheet. But in real world parties, they tend to be 2nd best at spellcasting, 2nd best at fighting, 2nd best as social interactions, etc, which translate into weak PCs when measured by the Drama factor.
