It's mainly a weight thing. The fcs fins are a lot lighter (the base of a MT fin weight a fair bit) as are the box installation. So it's a trade off. Another plus for the fcs system if of course availability which means that if you break one, is normally very easy to get a replacement.
But as you say, more trim options would be nice. Personally I think a new type of box with both lighter weight and some adjustability should be developed. The US but is good but just too heavy if we want four of them. The MT is pretty light and strong and easy to use, but lacks adjustability.
I don't think the fcs fins will break from landing jumps though. They are more solid than you might think and there are after all four fins to split the forces.