I always saw the Willow/Kennedy relationship in Season 7 as more of a parallel to the Spike/Buffy one in Season 6 than anything that Buffy ever shared with Angel. Willow, like Buffy before her, was recovering from a great personal loss and desperately needed someone to help her deal with the pain and disorientation resultant from that loss (Buffy losing heaven, Willow losing her own "heaven," Tara's love). Like Spike in the previous season, Kennedy supplied that love and support. Both Spike and Kennedy underwent great changes (soul/being "Slayer-ized"), after which the respective relationships evolved, developed, and strengthened, rendering each partner stronger and more self assured. Like Buffy before her, Willow was at first reluctant to recognize and admit her true feelings.
At the beginning of Season 8, we are told that Willow and Kennedy are (temporarily, of course) not together, but there is a reunion, showing a stronger more stable relationship. (Obviously, that part hasn't happened with Buffy and Spike...yet. But it's just a matter of time, and it'll be worth waiting for)