I'm not too keen on this option but it has more to do with story reasons than utility:
The only reason Astarion even offers the spawn transformation (in my opinion, it's open to interpretation) is because he has a great deal of love for Tav and doesn't want her/him to have the option of leaving him.
His fear of abandonment is big thing with his character and he just made a real romantic connection with a person for the first time in 200 years. So making it so they have no choice but to be with him forever makes sense for Ascended Astarion.
It isn't for tactical gain or strategy. It's purely emotional.
When he speaks of next steps for him and his new powers, his plan is to turn a bunch of idiot stooges that don't have a thought in their pretty heads into spawn so there's no threat of them doing to him what he did to Cazador. In that sense, unromanced spawn Tav (especially Durge version) would be his biggest threat and mistake.
Generally though, I'm all for people to have more choice so if this ever gets implemented, I won't be upset or anything. His motivations for the romance won't change; no harm done.
I just personally don't think it fits the character all that well.
