Dispensible: Arcane Casters

     Easily the most replaceable character type in the D&D world is the arcane caster. I mean no disrespect, but experience and logic tell me that most games can still be completed without having a mage-type (wizard, sorcerer, etc).
     First, let's think about what a mage does. They cast spells, and that's pretty much it. Still a very important role, to be sure, but that also depends on the types of spells. An arcane caster really only has two types of spells: damage and stat-effect (buffs, debuffs, etc). Sure, there are spells that can do all sorts of random effects, but those are almost never used. Leomund's Tiny Hut sounds fun, but what caster in their right mind would waste the spell slot on it? Let's face it, magic is prepped for combat, and not much else. There are spells that could help in social situations, yes, but the Face of your party should be able to handle that without help. There are spells that can aid in stealth, but the rogue or ranger should have that angle covered. Finally, let's face it, both of those situations can also be circumvented if your party isn't quite diplomatic or sneaky enough. Most of those situations involve brute force, but still, the job can get done.
     If you're looking for an effect-caster, it has been my experience that a cleric can fill this role just as well (if not better). Their spells are mostly designed for support: buff, debuff, healing, and summoning. A divine caster tends to have spells that help the party as a whole, not just the cleric himself. Not to mention, when the chips are down, a cleric has decent attack and defense capabilities (so if they manage to run out of spells, they can still hit things with a mace and hide behind heavy armors and shields). Mages, on the other hand, are just squishy sacks waiting to get the beatdown if they run out of spell slots.
     Where arcane casters corner the market is damage-dealings spells. And don't get me wrong, nobody can output as much damage per round as a sorcerer built for evocation. A single well-placed fireball or lightning bolt can deal more damage than any other single character. But if all you're looking for is a damage-dealing machine, is a mage really a better option than a well-designed fighter? 
     In one of my campaigns, we reached mid-to-high level (12-17). At the end, it was something of a competition between the warmage, the fighter, and the rogue as to who could deal the most damage in a single round. A crit with Power Attack on the fighter's keen falchion was on the same level as the rogue's 8 or 9 sneak attack dice and the warmage's fireballs. So it is certainly still possible to maintain the appropriate damage-dealing threshold with non-spellcasters.
     Also consider that, as far as damage-dealing goes, there are plenty of monsters that have spell resistance and resistances or immunities to energy damage. That means there are chances that a mage's spells are going to fizzle or deal far less than optimum damage. Sure, there's damage reduction, but by the time you get those monsters your main weapon-damage-dealers (i.e. the fighter) should have a magic weapon anyway, which overcomes 95% of all DR out there. Basically, there's a respectable percentage of monsters that are immune to fireballs or lightning bolts, but pretty much nothing is immune to getting chopped to bits with sharp pieces of metal.
     So, while a wizard is a welcome addition to any adventuring party, in my opinion if you have to vote out one of the four traditional classes (for example, if you only have four players), then the arcane caster should be the first one on the chopping block.