I'm a healer Merc and I use Mako most of the time.
I think its pretty sweet that with her & my combined heals, I can solo a heroic champion a level or two above me. It takes 10-15 minutes, but it can be done.
With Mako, I never die.
If I'm using Blizz & I neglect healing him for a moment and try and do a little damage, the battle can turn quickly against me if he dies.
It just depends on whether you want to be the one doing damage, or you want to be healing your companions.
I usually quest with Mako so I can do damage, and pop heals in PVP and Heroic / Flashpoint missions.