Because people believe that you can not love a villain for his cunning, ingenuity and cruelty, that everything must be "fixed". And I like villains, I don't like corrected versions, ESPECIALLY when it BASIS on romantic relationship... Believe me, this is quite common, and it looks ridiculous. Also you can be a hero in almost every game. But not in every game you can be a villain. This is rare.
Of course, everyone plays as they want. I just explained why I think it's forced and boring.
That would only be an issue if they force you to redeem Astarion, I'm with you %100 on gating characterization behind a romance plot.
The most interesting character quests in BG:2 were the ones where you had an influence on the alignment of your companion, if they didn't continue this in BG:3 I think it would be a huge missed opportunity