It's gonna suck if everyone on the PC end of things rushes to level everything to 20 now just so "it's done" and when all the PS3 players join in we won't have anyone "want to bother" with us who knows what they heck they are doing.
Would be easy enough to remedy, just release some new jobs along with the PS3 launch. Plus, you'll have all the other PS3 players, and probably PC players who were waiting for the game to get out of beta before playing it. So I don't think it'll be that big a deal.
And like others said, it depends on whether or not you have fun with it. Personally, I had tons of fun leveling with the old party sp system, and we all got to 50 then. Since they changed the sp system, I haven't leveled any battle jobs, besides taking thm from 14 to 20. It's just not fun to solo grind and do leves/behests exclusively for SP. So I'm not doing it. I play for fun, not stats.
I do however, have fun killing NM's, so I just log in to do that and help people out with random stuff. If not for that, I'd just take an extended break until they patched in more content and tweaked some stuff. So if you're not having fun, don't play. Come back later, or if the games really not for you, quit... But don't force yourself to play on the promise that it'll be worth it later. Even if they do change things to your liking, it will be more fun to level at that time then now anyway, so what's the rush? Whatever they add won't be going anywhere once it's here, and you can take your time getting to it. Just have fun.