I'm a little ambivalent on the Minuteman. We all saw how loathed Protective became as a trait in CIV since if you're on the defensive (i.e. your cities are being bombarded, which was the main strength of Protective) you're losing. Or at least not winning as much as you should be.
An defense oriented ability like Protective may be limited, but at least you can plan around having it. A defense oriented UU like the Minuteman (or at least a home-turf-oriented) is something that not only will be of very limited value in terms of leveraging the advantage, but also in terms of length of that advantage. You can't plan on having beefed-up defenses like you can with Protective, since you'll only have them from Gunpowder to Rifling (which, incidentally, are separated by one tech).
HOWEVER (and this is a big however), this may end up having a perfect synergy with America's SA. If Manifest Destiny actually has a marked impact on America's land-grab potential, which would likely result in having some decent holdings by cutting off adversaries' settlers, there's a chance that Minutemen could be instrumental as a holding measure in the perilous mid-game period of "I spent a ton of opportunity to grab this land and now I can't do anything until my cities get better." Of course, expansion doesn't seem to be as huge of an economic drain as it was in CIV, but it does seem like they're still keeping in some penalties for rampant land grabs to keep smaller empires from being totally nonviable. Being able to cheaply hold on to your territory when the neighbors get jealous of your vast holdings is probably exactly what the devs had in mind here. And depending on a few variables (how big is the bonus, how large will Renaissance America be, will the penalties for that expansion necessitate a holdout against other civs, etc.), they may be absolutely perfect for that role.
[edit]Come to think of it, the Protective trait and other defensive specials remind me of complaints a lot of Priests had in Burning Crusade-era arena: our best PvP ability requires us to die first.