I'm going to get metaphorical, I think the reason that we barely see Mr. Summers is becasue the show is in a way a viewpoint of Buffy's mind. Mr. Summers appeared a few times because he was still in buffy's mind as her father, but still not fully there becasue he didn't live with her and wasn't around as much. Now as the show proggressed, she find a new father figure in Giles, hence Mr. Summers only being mentioned passsingly, he was more or less somewhere in the back of her head. So Mr. Summers probably did appear off scenes just the viewers didn't get to see it because in her mind, Giles was hers and Dawn's real father. Alot of this is backed by the episode normal again, her dad does appear in that episode I believe.
Does any of what I say make sense?