Further thought...

I wasn't accounting for the second dps slot, actually. There won't be that much of a difference. You're right.
Each heal/tank pairing bumps two dps up the list. A fourth category would just leave you waiting on that queue to fill up, which would mean longer waits for everyone.

Alrighty then. Point conceded.
I'll just join a dungeoneering linkshell.
Happy gaming.