I find support skills are best when divided up between the group, rather than designating a role and leaving that person's character useless in single player. In any of our multiplayer arcade runs, I always pick Protect and the other person picks Haste.
They're far more valuable in Arcade than in Story Mode, but I don't necessarily mind some things being better or worse from Story Mode to Arcade Mode. The only reason they seem less appealing in SM is because you don't care if you get hit and you can already get your attack speed up fairly high with pets.
If you really are worried about someone being gimped due to focusing in a lot of Support skills, though, talents have a way of patching up any holes in a build. Something like:
Beefy Buffer - For every unique Support skill buff you have active that you have cast, gain +# ATK/MATK per charge level for it's duration.
That'd potentially make a support-built character stronger. You cast 4 Support spells, your entire party is properly buffed and you get to enjoy a tidy bit of extra damage for buffing yourself.
That's why I think Talents were one of the best additions to the game, really. If the skills available aren't allowing a build to be viable, Talents can easily fix that.
The Debuffer + Insult To Injury build is a lot of fun.
