Funny, when I tried it out I get:
Healing = kills just about anything that is not made for a large group.
Damage = My companion kills about anything with good DPS
Tank = ....... Eh, is this really tanking? Low damage compared to other roles and the healing strike does not heal all that much on tank. Damage mitigation is rather low as well. Can not hold area aggro.
In defence of my tank companion I should add that I mostly play gunslingers or snipers, so they have a lot of aggro to handle. Most companions can handle single target aggro reasonably well (with exception of the Nexus that is broken in tank role).