my experience as a Vengeance build Juggernaut:
I always rotate two to three depending on the situation. (Quinn, Willsaam, Pierce) (heals, DPS, tank respectively)
Every situation has a useful comp.
mostly I travel with Willsaam geared DPS.
(she helps take down average mob groups very quickly).
and whenever tougher mobs or a boss comes into play I pull out Quinn as healer.
(if I feel lazy I run with him too, he heals constatly...dont need to channel at all)
I have the tank, Pierce, but I admit I dont use him too much yet as I am still gearing him up.
( you need to gear tank companions well for them to be effective I have found .... or even survive any amount of time ).
(( I dont travel with Vette much at all, she is a crit crafter in Treasure Hunting...so I have her away almost all the time. ))
I just picked up Broonmark (low 40's in lvl I am), gonna re-gear him for DPS instead of a tank ( that way I can save some cash and give him my hand-me-downs )....he will be fun to chum around with.... I'll switch between hin and Jessa for everyday grind leveling from here on out.....until I need the medic....or the tank.