IMHO, the best spellcasters always cast with an eye toward the party. While it's useful to turn yourself into a fighter for a few rounds, it's normally *much* more useful just to buff the fighter (or even the cleric), and have him protect you.
Self-interested spellcasters who insist on protecting themselves above all ultimately fail, because one Dispel Magic and they're useless.
Sure, the cleric CAN be quite a machine if he wants to be. But that leaves the rest of the party high and dry, and when his spells run out and his opponents dispel his magic, he's a sitting duck.
Meanwhile, if he turns the machine into a BETTER machine, not only does it help the party in general by making the encounter less difficult, it helps him specifically by sending someone with higher hit points into the middle of the fray, and a Dispel will only slow them down, not stop them cold.
It's kind of mathematical: would you rather add +1 to 0, or to 1?
On this scale, normally Wizards are at the far 'preserve self' end. They don't have a lot of spells, and those they do have are geared to blowing stuff up, which tends to piss off anything that survives. Wizards feel the most frail, so they're the most likely to buy a wand of Mage Armor instead of a wand of Magic Missle. Though even this adds to the party by softening up the enemy, and protecting one of their most unguarded flank -- the spellcaster.
Sorcerers are slightly less than wizards, because they have so many spells...they can be just as frail, but they tend to have more staying power. They'd buy a wand of Magic Missile simply because they'd cast it all day sitting behind their Mage Armor that can be put up again and again (Having more Magic Missiles is more useful to a party than having more Mage Armors).
Druids fit about on the level of sorcerers, mostly because buffing their animal companion is pretty much like buffing themselves, and they tend to focus on the destructive divine magic and melee powers.
Psions tend to be the lowest of the 'preseve self' end, because although they often feel quite frail, their versatility ensures that an entire party will benefit almost as much as they themselves will.
Bards balance the line a bit, but tend to support the party over themselves. Heck, even Bardic Music is geared for that. They're not the soft targets wizards and sorcerers are (though they still ain't tough), and they can enhance things almost as well as the cleric. Plus, when their magic works, it's nearly insta-kill: a Charmed monster doesn't resist.
Clerics sit firmly in the 'party benefit' camp, because, again, giving someone with a BAB of +5 another +1 bonus to hit is going to be a lot better than giving someone with a BAB of +3 that +1 to hit. Same things with AC's. Saves are a bit iffy, since people like to shore up their weaknesses here, but same principle. They have a few specific self-benefit spells, but I've never seen a cleric use 'em...they're too busy healing and giving +1's out to worry about entering melee with the combat monsters. Instead, they'll help the Wizard or Sorcerer or Druid out by making them less of a target.