Aside from the balance issues mentioned above, specs work in other games because without them you're committed to playing as a tank/healer/dps/support for 100% of your character' life. I think part of the intent behind the "level all jobs" aspect found in this game and FFXI is that if you want to play a role different from your current job, you can always switch to a different one without having to make a new character. I believe this has been reiterated by the devs at Q&As as the intended alternative to multiple specs.