How many NPC wizards do you have using Dimension Door to get back to their bedroom and Teleport to escape after picking up the emergency bag? Many GMs would have the NPCs use those slots for yet another attack spell.
It is because of this fatalistic viewpoint, where they never worry about tomorrow because they never expect to get there, that makes the balance different.
Bret, I agreed with your post except for these points. My wizards always have an escape plan, usually one or two Teleport spells. (The second in case the first is interrupted) and often a Silent Dispel Magic.
However, in terms of blowing away all their resources in one sitting - yes, it's true they can and will do this. Even if the wizard plays like a PC (eg hoard those potions for the next battle) they have no reason to hoard their spells. As long as they Teleport to a safe place it doesn't matter if they have virtually no spells.
As for wizard magic items being worse: take a look at a wand. In most cases the wand will have a lower save DC than anything the wizard could cast himself. A Wand of Fireball is nice (50 Fireballs, can use 3rd-level slots for something else) but with a save DC of 14 the party has little trouble with it. It's isn't possible for the wizard to use his own Int bonus (perhaps backed up by a Headband of Intellect) to boost the wand's power.
On the other hand, take a look at the fighter's items. Belt of Giant Strength (bonus to hit and damage), Magical Sword (bonus to hit and damage), etc. Those items are, effectively, stacking with each other and with the fighter's Strength score.
An NPC wizard will likely have lower AC and saving throws than a PC wizard. Boosting the AC is easy, however - cast Mislead, Mirror Image, Displacement or anything that boosts your defenses - they will have a bigger effect on the wizard's defenses than his magic items. Sure, a Ring of Fire Resistance 30 is cool, but it is no match for Protection from Energy.
Of course, the BBEG's NPC disadvantage starts to fade when he gets hit by Dispel Magic, but if he readies a Time Stop or some other combat-breaking spell, it's game over.
PCs can expend their spells if necessary too though.
True, but they won't.
There are usually two ways of fighting a BBEG.
1) It's the first fight of the morning. If the PC wastes his spells, then he will die in the next couple of encounters.
The PCs can't always "call it a day" either.
2) The fighters had to run the gauntlet, either marching up the main hall and wasting resources against many lower CR opponents, or they took the secret passage with alll of it's traps. By the time they get to the BBEG (who is trying to run out of another secret passage) they have used up a lot of resources, and they have to fight the BBEG and his bodyguards. This means that the PC cannot cast as many spells in that combat if it is drawn out for any length of time, assuming the PC spellcaster is the same level as the NPC spellcaster.