OK,
If you harvest them you get full adam, allowing you to purchase many talents and upgrades such as more health e.t.c.
If you save them, sometimes, not sure when but you normally get packages to say thanks. These come in the shape of health, eve and such. Pretty handy although normally can be found the levels. However in some instances you get quite a bit of adam. Ive had +200.
Either way you can't really go wrong. However if you don't harvest any of the girls, a different ending awaits....
Gonna have to reply this game after not harvesting the girls at all and see what happenes, sounds exciting