I would say that chance of surrender is relatively low, or maybe they die too fast. You would have to try to ping them with something like single gauss cannon to try to force more of them to surrender before you damage them too much and they explode.
At the end it is not really worth it IMO. They will most likely not fit with your existing designs anyway. Only thing that may be useful is to get colony ship with colonists, so you can start second species in your empire.
I sometimes capture ships by boarding them but you need good speed advantage for your marines not to die during the assault.