It didn't bug me, but it was a little strange seeing someone clearly supposed to be Buffy, and them using previous Buffy voice overs, and it...not be Buffy. Probably pretty cool for whoever that girl was though. Even if she never acted again, she can say that for one episode she got to play Buffy the Vampire Slayer (well, kinda).
As for the larger question of SMG in S5, yeah, I thought she should have been there at least once or twice, and more if it had been possible. I know she was trying to break out of the "Buffy" mold (which obviously didn't really happen), but it was just such a shame that she didn't do any Angel guest spots. Who knows though, that might change in the future. She's always spoken very highly of Joss in interviews, so I don't think there's any hostility between them, and she's said she would play Buffy again, if it was the right fit.