As many members know, I'm a HUGE Buffy/Angel shipper, and often even go so far as to rewrite the storyline through ficlets to try and keep his character with those of BTVS. Out of all the characters on btvs, he was the worst to see go, for me.
But at the same time, I don't think it would have worked, having him around.
As much of a fan as I am, I really think looking back that he and Buffy really both needed to grow up (Buffy in general, him in growing into his humanity, rather than hiding behind a "creature of the night" persona). His leaving meant that Angel was forced into a new and independent role as a "hero" on his own-- with friends around him that served as a very humanizing force. Look at Cordelia, Wes, Doyle-- all of his friends forced out SOME aspect of humanity that even after his years with Buffy had been supressed. In Sunnydale, Buffy was really his only signifigant tie to humanity (and the only person that he had a friendship with for that matter, which isn't healthy for anyone).
And then we have Buffy. Parker, Riley, Spike-- all those guys taught her something about life (and herself) that I don't know if she'd learn with Angel around. Would she learn what it's like to have a guy use you? To maybe/maybe not be in love with someone as much as they want you to be? To sleep with someone out of depression, rather than love? All were experiences that drastically changed her character and forced her to grow up. And muchlike Angel, she for a lot of the later seasons was independent-- standing alone on her own two feet. Which again, I don't know if she'd ever learn how to do with him there (obviously slaying aside, I'm talking about being an independent human being).
Also, look what went on. The Initiative could have used Angel against Buffy, or Cordelia would have been dead in LA somewhere (probably most of the characters on Angel would have been). And the Dawn situation; when Buffy sacrificed herself on the show her mom had died, three guys had used her so far, she had found out that her sister "wasn't real" and had spent months trying whatever she could to protect her AND deal with being the head of the household-- although it was still a difficult choice, she saw it as "her time" to let go. To die. I can't help but wonder how that situation would have ended up if she and Angel were on year 5 (on again off again) together. Or if he would have tried to intervene in some way.
I don't know, it's just the more and more I think about it, the more and more I think that maybe it was best that they were seperated. I would have liked to see him come back in season 6 or 7 (or even 8, for that matter... pretending the spinoff didn't exist), but I think him STAYING the whole way through would have in the end been bad for both characters. Even if not for the reasons that Angel gave when he left.