See, in 3e there's a basic assumption that an encounter between four 5th-level PCs and one CR 5 monster should drain away about 25% of the party's resources, which primarily translates into spells (and primarily the cleric's spells, which determine everyone else's total hit points).
And it's a flawed assumption. I don't know where they're getting this idea. Firstly, a CR 5 monster is an EL ~3 encounter. Secondly, I don't think a party would blow more than about 15-20% of their resources against an equal-EL encounter, unless things went really badly for them.
I have to chime in on the side of folks like Sun Knight - if you're blowing all your spells in one or two encounters, maybe you should rethink how you're playing your PC, because realistically, that PC wouldn't survive very long. Wizards are supposedly smart - high Int, right? Clerics are wise. A wizard would look at the battle and say, "Should I cast a spell now (this round)? Or should I do something else? Should I just lean on my staff and watch the fighter hack the monster to bits and offer sardonic advice?"
'But it's not FUN!" you cry. "The wizard's just standing around back there twiddling his thumbs while everyone else does the work!" Well DUH. He's a leader. He's a tactician. He's a controller. Call it what you will, he's
supposed to stand at the rear of the party (or in the middle). Our group had a skill called Knowledge (tactics) - you make a check, and you could offer advice to the other PCs that would gain them benefits - kind of like the teamwork feats from PHBII, but we've been using them for years. That's what someone with easily the highest Int in the party would (should) have. Personally, I wouldn't have a problem with giving them AP Light and a single martial WP of choice - the sword-wielding melee mage is very common in literature. They'd still have low hp and BAB, so they're no threat to the fighter, but they can defend themselves and others.
Clerics and druids have the hit points and skills (armor and weapons) to mix it up with the enemies - they can serve as first- or second-line combatants. Bards? They're support. They aid their comrades by singing, and they have roguish abilities - they can use their bows as archery support, or sneak around and flank enemies for the fighters,
while singing. Sorcerers? They're strictly artillery. Given the fact they have a larger number of spells than the wizard, it's unlikely they'd run out before the party needs to rest.