The only draw back would be limiting the amount of jobs to come out in the future by attaching them all to current classes. In theory it'd be good, but they're really just extra skill sets are they not? Nothing is stopping a Pug from evasion tanking, and in fact they can be quite good. But the job attached to Pug, which could be monk? Would allow the Pug to deal more damage with attacks that don't rely on evading attacks as it does now.

So in theory, Pug is equipped with the skills that rely on tanking, so the job you unlock would be maybe for more of a DD. Gladiator is a good tank as well, but perhaps as a Paladin you rely less on strength and more on white magic and protection of others as well as yourself?

I don't know, speculation can be fun. But if they're being referred to as alternate play styles, I'd kinda like to think that the jobs are covering what the base class can't do well, or enhancing what they should do best.