The problem is that they are no longer mere side chicks.In a few words: too many pointless side chicks in the DIK route.
Sandy remains a side chick, if we don't see her in EP10, who cares. On the other hand, if we can choose Nicole at the end of EP9, it means that we will see her in EP10, and not just for a quick sex scene.
Rusty's reaction isn't because of Jill and their past.I remember this scene but I also remember Jill telling MC that she had a date with Rusty, but at some point Rusty's friends arrived and he acted as if she did not exist at all...
That's because it's the only walk of shame scene where the MC clearly tells the DIKs that it's his girlfriend, and therefore it's a serious relationship. The kind of thing Rusty was hoping for with Micha.