What in particular about them makes them good for this role?

It seems to be a bit of additional punch to the space weapon and repair skills, and some somewhat heftier bonuses to a few of the space science skills. Whereas on the ground they have some resistances and fast regeneration (which I am not sure has much of a benefit). That said, the tanking abilities are one of the smaller boosts.

It SEEMS like a Joined Trill would be best for a science toon with the size of those other bonuses... or actually, perhaps more accurately, those commanding science SHIPS.