I have two smarts and my father has one too, so that would be fitting a four-down towing setup on all the car$. Also, if I decide to upgrade to a different smart for whatever reason - once again no hassle of fitting it with a Blue Ox whatever your choice is.
Also, correct me if I'm wrong, but you have to have the key on the ignition for 4-down on the smart. On the trailer I'm not bothered with having to remember the key in the car if I stop while driving the motorhome.
Next is with a hitch ON my smarts the trailer I had looked at (same one?) weighed around 440lbs so if I find something I want nearby that won't fit IN my smart I can hook up the trailer and tow it behind the smart to get whatever I want/need.
Next up - I can always use an extra trailer

. A utility trailer is a nice thing to have around. A few stake pockets and you're set, or you don't even need the stake pockets if you don't want them.
Finally, when it's all said and done and you don't want your smart anymore you can keep the trailer or sell it for a pretty decent amount as aluminum trailers don't really depreciate too bad so you can get a good amount of your money back out of it. You could get your money back out of the 4 down towing setup but of course you'll need to remove it. (I know you could try to sell the smart to someone who wants it for use as a dinghy, so that is a thought too)
Of course those are the reasons I choose and like you said, everyone is different.