Wizards (and I'm talking about all wizards other than 4th Ed D&D) are both a boon and a bane. Anyone that has played a wizard from level 1 know how difficult it is to survive and be a productive part of an adventuring party with the limited number of spells he is allowed to begin with. With only 1 or 2 spells available early on, you pretty much get relegated to being a crossbowman for much of the adventure. A party of adventure hungry warriors and priests don't want to have to stop for the night after every encounter just so you can get your sleep and magic missile spells back. And I can't blame them, so it's back to using the crossbow.
However, at a certain point around 5th or 6th level, the tables begin to turn. Suddenly the frail, waif -like wizard that everyone has to protect like a party of big brothers becomes a man. Suddenly, that one or two spells becomes 2 or 3 or 4 per level. Suddenly you have FIREBALL! While technically not the best spell in the book, it is the crowning achievement in wizardry. It's the spell that lets the rest of the party know that you're ready to play with the big boys, even if they don't realize that you have better, more useful spells in your arsenal.
So here's the question: once you've arrived and have a nice selection of spells in your spell book, do you become a damage dealing juggernaut of pain and sorrow, or do you play the crafty backseat tactician of the group?
Both have very clear advantages. First off, dealing damage is just plain fun. There's nothing like looking down at the battle map and realizing that there are 4 plump, juicy gnolls in a nice straight line and you happen to have lightning bolt memorized for the day. I think the party can appreciate the input you give when a volley of magic missiles come darting in from the bleacher seats or when your signature fireball detonates in the middle of your enemies and all that's left are 3 pillars of orc shaped ash. But isn't straight forward monster murder what you brought the fighters along for? Isn't it their basic goal in life to rend and tear/ hack and slash? Isn't it overstepping your boundaries by doing someone else's job?
Or is it better to be a support-type wizard that does the intangibles? Wizards are in a unique position to do things that no other character can do, shouldn't they be doing it? You don't expect a fight to trap 3 bugbears in a mass of sticky web do you? It's not like the cleric can make someone climb on the ceiling like a spider is it? And the thief isn't going to conjure up a rolling mist to hide the party from goblin archers either is he? So who's going to do all of that if not you? No one, that's who. It's the wizard's job, no, it's the wizards responsibility to think outside the box. Don't just scan down the list of damage dealing spell to see which ones will impress the party most. Think about what will keep those pesky archers at bay while the fighters do their job. Think about what will help the thief scout the rooms up ahead so you don't walk into an ambush. And most importantly, think about what will help the party stay alive so the cleric isn't just a walking first aid kit.
Everyone has their own play style of course and some people like being the nuclear bomb that the party breaks glass on in case of emergency, but I find it a thousand times more beneficial to the party to be the unexpected support. I've played both ways, and personally I've received far more compliments for helping the other party members do their jobs than I have for unleashing hell on the enemies. There were many an adventure that I would load up on damage dealing spells and when things went south, the party had no way of slowing down an enemy or halting reinforcements long enough to make a tactical withdraw. And on the other hand, there have been times that I've saved the entire party by webbing up a doorway so a flood of enemies couldn't get at us until the cleric could help the warriors, or a bottomless chasm was easily, and safely, bypassed because I didn't memorize 4 magic missiles, 3 acid arrows, 1 lightning bolt and 1 fireball.
Sure, you can't possibly think of every situation that the party might get itself into for the day and prepare spells to cover them all, but isn't it more fun and challenging to try rather than simply going with the same ol' fire and brimstone? Isn't it more fun to see that frustrated look on your DM's face when you walk right through his carefully planned trials and pitfalls because you were well prepared than having the party all stare at you waiting for you to do some wizard-stuff to help out? In fact, you may even be rewarded for clever thinking and outstanding role-playing if you're not just a one-dimensional wizard-grenade.
And well all know reward = exps and exps = more spells!