I have noticed over the years that even if the Wizard is powerful and can hold his own the player will tend to sandbag on the spells for 'just in case'. By the time the heroes get to endgame the mage is still flush with spells.
What I have seen as a pattern:
First couple of fights the big guy and midget take out the monsters as the group is smart enough to be in some semblance of a formation. Maybe one of the fighters is hurt but not bad enough to heal and the party is worried about the future events so spells are reserved for later.
As the dice are not always friendly and exploring happens the formation is totally messed up a few turns in, perhaps the group is even separated by now. The spook and ol' pointy ears get into melee without the 2 fighters eventually. Or if the movement dice are really rude, the spook is alone when he is jumped by a greenskin. If the mage does not die then the healing spells are used to heal his sorry but.
Now the group or remainder resolve to regroup and travel on.
Formation holds for a turn or two then the heroes encounter another fight. Warriors dispatch the bad guys and the spook probably throws a damage dealing spell as he feels kind of guilty about being so useless. But he knows if he can just find the end boss, he will melt that dudes face in a flurry of arcane wrath. Right about now he probably wants to unleash that wrath on the midget.
So they either split up the party due to ego issues or the ego issues are resolved enough to get over it, they reform and soldier on. If they split up they die. They finally find the end boss, the midget and the big guy storm in and deal some heavy damage. The spook is thinking 'YES My time to shine is here'! But it turns out he either can not get a good line of sight or worse he fires off his most powerful spell to have it just come short of killing the boss. Now Mr. pointy ear waltzes in and sends his one really good damage spell downrange at the softened up boss and the boss dies.
The spook still has a handful of spells in hand and his moment of glory was stolen by the pointy eared dude.
Or the spook is played by a player who doesn't understand the need to be selective of spell use and by the time they are halfway through the dungeon the mage is left with a single spell in hand that he has found no use for. Now the spook is just running around and dagger stabbing greenskins and hoping for the best. The other 'Dupes of Mentor' are left scrounging their gear for any spare scrolls, and potions in a hope to keep the spent spook alive and maybe armed for the end boss. They yell at the dude for wasting spells and admonish him to save the scrolls for the end boss. When they get to the end boss the spook doesn't realize it and he keeps doing simple dagger strikes. By the time they get the spooks attention to the fact this is the endgame room and they have him on board to finally cast that scroll the midget gave him, the big guy gets in a deadly blow and the spook is left holding that one good thing that could have shortened the battle substantially.
Scenarios by Cyndi

So it is said that if you know your enemies and know yourself, you can win a hundred battles without a single loss.
If you only know yourself, but not your opponent, you may win or may lose.
If you know neither yourself nor your enemy, you will always endanger yourself.
~Sun Tsu The art of War~