The Healer is a good companion because his heal is quite good and saves you from using heal potions between the fights and saves you a lot of gold. But the Healer has a low survivability and will die in many boss fights.

A secondary companion may be useful. Especially the Dog, Panther or Wolf because they have abilities to knock down/disrupt enemies, have a higher survivability and do some damage. This makes them better for boss fights.

I do not suggest the Tank. He may tank some mob but overall he feeled quite useless. Galeb Duhr may do better but I never used him.

Some more information on the companions:
http://nw-forum.perfectworld.com/showthread.php?111111-Companion-info-and-a-bit-of-a-guide