What are the goals/specialization/actual use of each?

FreeBSD: I always got the impression this was trying to be full modern UNIX but non-linux

NetBSD: I guess this is for older/less powerful computers based on comments here?

OpenBSD: ???Security???

Dragonfly: a schism over threading, but FreeBSD?

Every Linux distro has different goals. But a unified kernel (more or less).

For hardware, can a single device driver be made for all variants of BSD? If so, then I agree.

You aren't going to see OpenBSD share a kernel with anyone - it's too different and makes trade-offs the others won't accept. And NetBSD doesn't need the heavyweight kernel FreeBSD uses.

From what I've seen, the BSD community swaps code around on a regular basis. But they pick and choose what code to use based on their own goals. It seems to work pretty well.

I know nobody will touch this with a ten foot pole, but from a thousand mile view the BSDs seem like a good candidate for a microkernel.

There's a lot of shared code.