I think the biggest problem healer companions have earlier on in the game is that they generate too much threat with their healing abilities. It was fine for me and my run through with my Vanguard and Elara since I could easily generate enough threat with ion cell and then in later levels, taunt anyone who came near her or looked at her funny...
In your case, chainz, I think you'll find that Quinn (when you get him) will probably end up coming under fire quite a few times and when that happens, he goes from healing you to healing himself... which can be troublesome for you if there's still enough attention on you.
I'm pretty sure Astor was having that problem since he's been using Quinn through his run-through... except that one big fight at the end of chapter 1 where I got him to switch to Vette
Of course, that said... I think the dot could help you generate more threat, but the real clincher is the fact that you don't have any taunts. I mean, the Marauder wasn't made to taunt at all, it was made to stay out of trouble which is why it has Force Camouflage which takes down your threat level for a little while.