Well there is actually a lot to gain.
The male has a limited lifespan after maturing anyway, so might as well risk it for some potential replacement slings.
The owner of the male doesn't do alot after sending the male off. The owner of the female has to worry about getting conditions right for breeding and trying to convince the female to drop.
I personally don't do breeding loans with anyone (except for a few people that i trust), because people are always out to make a quick buck and money seems to talk more than ethics and morals.
I have been screwed over, and a few close friends as well, from owners of females claiming that the male was eaten before mating and then sending my male out on breeding loans behind my back. Or saying that the female moulted out only to see slings appearing on the market a few months later.
I refuse to send males out where I cant see what happens to them after mating attempts, and insist on keeping the females to make sure that the owner of the female doesn't try pull a fast one on me. This thread could be directly linked to the 'people' thread on here actually.