The point is that for Goris to level up - you have to level up, but by the time you encounter Goris you have (most likely) levelled up so much, that you probably only have a few more levels to expect, making the effort a bit wasted. In comparison, Vic or Sulic who have been with you from the start, have levelled up alongside you this whole time.
(your followers only level up when they are in your company.)
Another slight downside with Marcus is that he litterally doesn't let you boss him around. You can customize the combat tactics of your followers, and each have their limits and preferences. Marcus likes to charge head on, and won't be told otherwise.
What weapons your companions can use is limited to what animations they have. You can ask them their preferences, and try and guess around a bit. I would recommend, as alec said, a laser or plasma rifle for Marcus, because he 1. won't kill you every time he tries to help you (miniguns) and 2. won't charge face-first in for a melee fight that easily gets him killed, cus he's a fight-to-the-death type of feller.