I'm not a mum, but looked into being a surrogate because I felt guilty choosing not to use a (presumably - judging by my ridiculous number of first cousins) healthy womb.
A similar argument is made about eggs being released each month and we are just wasting them. How selfish of us women throwing way such precious gametes/DNA when they could be 'used' by someone else to have a family. Terribly careless of us.
This is an interesting discussion OP, for me each if the women who were surrogate mothers in the BBC documentary series had some kind of mental health/emotional void they were seeking to fill. I saw each of them as vulnerable. Faye, who has Baby Miles for single gay man David spoke about it briefly when she was walking her dog and Emma, the single mother living in a studio flat with her 2 year old son, having Baby Mia for Kevin and Aki, she said she would be devastated if there wasn't an enduring relationship. I think she wanted more of a co-parenting situation too but that's just how I saw it.
Whether these adults do bond for life and stay in touch remains to be seen. I read a lot to suggest it's not as it seem, or as we are expected to believe from mainstream media coverage on surrogacy.
Donor conceived children who will be able to know their genetic parents as the law changed (2005) are hitting 15/16 this year, but I imagine that it could be another 10-15 years, when they maybe begin to consider starting a family of their own, before they think of where their DNA comes from, if they haven't considered looking before then.