6 box group is commonly Tank(gold/sub) + 5 FTP toons. Some people will pick and choose a few other toons to sub. Melee with good prestige weapons can get quite a bit out of gold. You can also consider buying the Rank II spell/ability upgrade for FTP in the marketplace for 750 Daybreak points.
Tank - AC = You get to use the best armor and augs. That's really the big bonus. You can build a good FTP tank, but sub makes it easier and opens up the top level T2+ gear.
Melee - The T2 or chase weapons can be worth the DPS boost. Plus the +dmg prestige augs.
Caster - You're not getting much unless you want the cool dmg procs from some weapons or quest/raid gear. I think the rank II upgrade provides more DPS than the other bonuses from sub.
Healer - Definitely a good one to get rank II upgrade, though sub doesn't get you too much more. I think there are some prestige clicky heals though like VP quest one.
As mentioned above, people tend to burn a Krono on each toon once they're at max level and try to get all the AA before it runs out. So you keep all your AA, but don't need to stay subbed.