Taking the RER/metro from CDG to Gare Montparnasse is probably a lot more streamlined and more easily navigated than getting to a hotel.
Not necessarily, because getting to Montparnasse requires a change to the metro, and Montparnasse is on the far side of Paris from Gare du Nord, where one arrives from CDG.
It's possible that going to a hotel you might be right on the RER B line, or along the RER A line, which is a cross-platform transfer. So if you were staying along either of those lines, I would judge that as easier than getting to Gare de Montparnasse.
It's a shame that the Cars Air France don't exist any more. That was a nice "in-between" option for getting to Montparnasse.