The other nice thing about the tier system is that it helps prepare DMs for what players are capable of. This is as true for an unoptimized player as it is for one with near-total system mastery.

As an example, the first character I ever played when 3.5 came out was a sorcerer. I was in a game with an archer ranger, a Power Attacking fighter, an OA-heavy monk and a cleric who ended up dipping a level of fighter because cleric just wasn't cutting it for him.

My sorcerer was not a very well-optimized PC. I wanted to play a very shaman-y character, with a lot of minor charms and just about anything animal-themed I could find. Most of my feats were skill-focused. The only really good spell I had was Sleep, and even that didn't stay good for long.

That is, until I hit level 8. I'd been keeping up with my animal/charm theme, and the first spell that caught my eye in 4th-level was Polymorph. 3.5 was brand new and I'd only played in one 3.0 campaign prior to that, so I didn't know anything about all the headaches and power disparity Polymorph caused.

So I took Polymorph because I thought it was cool, and the first time I used it, I flipped through the Monster Manual to figure out what to change into, when I stumbled on the remorhaz.

So, in my first combat session of 8th-level, I went from being the mostly useless sorcerer whose primary contributions tended to involve bluff checks to a 20' long multi-legged beast of fury. I completely overwhelmed the first couple of encounters, and was consistently out-damaging the fighter and monk, who had hitherto been our primary melee tanks.

As I kept messing around with Polymorph, I found a lot of really cool and unintentionally abusive forms to be, and I ended up completely overshadowing the rest of the party and frustrating my DM. Not because I was a min/maxer, or because I had any great system mastery. Not because I built to be a power player. Just because I picked a spell I thought was cool, and the DM hadn't ever given much thought to the idea of using Polymorph for combat.

I can't help but think that the DM, if he had been aware of the tier system, would have recognized the potential tools a sorcerer might have at her disposal, and been better prepared to deal with that, or might have directed me away from spells like Polymorph.

It's the same thing for wizards or clerics. A wizard can play like a crappy archer for 3/4 of the game, slinging around damage spells that don't even compare favorably to the fighter's greatsword. A cleric can spend his whole time healing. But all it takes is one day where the wizard player thinks, "hey, this Solid Fog spell seems cool" and upon using it realizes that it completely locks down 75% of enemies in the SRD, or one day where the cleric decides to prepare Find the Path on a whim, and inadvertently bypasses the majority of the planned adventure, for everything to change.