I see what you're getting at, especially about the namespace conflicts. However, I'm hanging up on this: "most of the logic related to a dubins cart should and will end up in the libraries code anyway. Your plane follower that takes care of taxing versus flying is going to have enough logic dealing with that distinction you'll have the calls to the dubins cart in totally separate blocks of code anyway." 

I haven't tried to implement the two together, but I have my doubts about how efficiently this can be done. If this were clean to implement, then that would effectively mean that we wouldn't need a separate dubins cart path follower at all, not even for cars since they could just use the fixed-wing flight follower.

I also see a problem with the idea that the fixed-wing path follower will be handling this logic. It's not really its job to be the hybrid automaton. The logic to switch between path followers needs to occur at a higher level in the flight director. Otherwise the path follower becomes needlessly complex. I realize it's not an argument for or against merging the followers, but we need to make sure followers have a narrowly defined task of following a given path according to mathematical rules.