I think I've met a good number of belt chasers in my time. I'd like to say I've seen more than my fair share but then I'd probably cop some stick the next time I go to class. Once I was talking to this guy who trained in Wing Chun, and he actually said "I can't wait to get my black sash, cause it'll be, like, my walking papers".
I suppose if you're training in a martial art which places less (if any) importance on functionality, it's safer to chase belts. If all you're doing is kata and zero-contact point sparring, then fine, I can kind of see the rationale behind it. I'm sure we've all got a story or 30 of some guy who was a black belt in such-and-such martial art who totally didn't deserve it.
If on the other hand, you train in something like BJJ, putting yourself at a higher level than you should be just doesn't strike me as a good idea. First of all, as soon as you start rolling with someone they're going to have a pretty good idea that you aren't what you seem. It's quite a potentially risky thing to do.
Case in point: most BJJ gyms that I've seen have some kind of prohibition against leglocks - say, not until blue belt. So what happens when you walk into a gym wearing a blue belt that you shouldn't be wearing, and some guy puts you in a straight anklelock. You freak out and roll to the side, bam - you just heelhooked yourself.
Then, there's also the pride factor to consider. Walk into a friendly gym, and most everyone will be chomping at the bit to roll with you, because they're just antsy to roll. Walk into a competitive gym, and everyone who you outrank may just be salivating at the thought of the fresh meat to be had on the mats.
The potential damage to the ego of someone who needs a differently-coloured belt to feel secure could in either case be devastating.
So unless you want to be "that guy" - unless you're prepared to be the guy who sits in the corner, forever politely declining invitations to roll because "oh, I got a bum knee" - unless you want to spend your BJJ life being nothing more than window dressing - just say "NO" to belt chasing.
This post does not reference people with legitimate injuries in any way. Any resemblance to any persons is unintentional, except of course where it is intentional.