Who says it's worse? It could be equal, yet cooler.
Like 'oh hey you got x kills now you're allowed to use the sniper class' etc.
Generally the idea of a level system is to get people to play longer, as they want to work towards something.
If it's not an improvement then no one gives a stuff.
No one's going to stay on your server for an hour to rack up enough kills if all they get is a reskin of another gun or some stuffty effect.
Other classes could be a good idea, but the higher classes will either be overpowered or not worth using.
Imagine if TF2 made you start as a soldier and earn all the other classes. Pretty dumb, right?
Trust me, I've hosted this kind of thing before.