..One HUGE advantage to using a trailer to carry your car (enclosed or not) that you didn't mention:
Two words: BACKING UP. You will be able to back up the MH and trailer if you get in a tight spot somewhere.
When flat towing with a tow bar, backing up is very difficult or impossible. 'Tis the one bad thing about flat towing a vehicle with a tow bar.
I've thought several times about going to a trailer and doing away with the whole tow bar and flat towing thing. Never have done it, though, and probably never will 'cause I just can't get past the logistical nightmare it creates for hitching up, unhitching, storing the trailer at campground, etc. Not to mention the upkeep, extra expense of having to license, insure, and maintain yet one more set of wheels (trailer). Nope, 4 down is much easier and simpler for us.
Agree 100% with previous points - If you do go the trailer route, you'll make it much easier on yourself if the vehicle you carry in the trailer is capable of towing, moving the trailer around if need be. Definitely got to check on total length and apropriate laws on such, too, like already mentioned.