I like the idea of building your own bots. The advantage of this would be to build your own companion, and if that companion is killed you could craft another. I also like the idea of reprogramming a robot. Sneak up behind one, rip open it's panel, and turn that robot on the others. I don't like the idea of reprogramming a robot as a full time companion, but a temporary companion, to help clear a building or area would be cool.
I wouldn't want to build my own companion. This would be an option for dlcs in which you cannot bring companions. Also, these thing should not be able to last forever. Because they are improvised devices, they're power source could be unstable and they shut down within a certain time-frame, but if left with ample parts (junk) you could make more. The turrets could have weapons similiar to the Compliance Regulator, but less powerful, and given a certain number of shots.
I always wondered why we couldn't reprogram the robots like the turrets (with the exception of a certain quest in F3; that was fun).