When Spike is around Buffy he is not fighting so much as trying to impress her, and for that reason alone he is a liability rather than 'partner'. Moreover when souled, he usually slips and gets one of them injured, exactly because he has such a huge chip on the shoulder (about his helpless crash on her), that he cannot truly concentrate. Which is ultimately very funny , because when he is with Angel, or with Faith he is a much better fighter.
But then Spike is still at the bottom of this pyramid.
When in Season 7, did Spike "slip and get one of them injured?"
They fought the worm/man at the beginning of the season, but the only injury that I recall was the guy getting stabbed in the shouilder, something similar to Faith stabbing the Assistant Mayor in Season 3, but with less fatal results.
They fought in the alley when Spike saved Robin Wood. That MIGHT be seen as a slip up, considering the trouble that Wood is going to cause him later, but he had no way of knowing that at the time.
They fought in the abandoned Initiative Headquarters, but thi chip went off unexpectedly. I can't see as how that could be called anysort of slip up on his part.
They fought Caleb in the vineyard, and he beat the crap out of all of them, but I can't see that as any kind of slip up on Spike's part.
They fought in the ubervamp chamber in the last battle, but I don't recall any slip ups there, unless staying behind and destroying the ubervamp army at the sacrifice of his own life to assure that Buffy and the others get away safely. Is destroying Sunnydale while saving the rest of the world considered a slip up?
It's been a while since I've set down and watched Season 7 in its entirety, but those are the only times that I can remember when Buffy and Spike actually fought together that season, and I don't recall any slip ups by Spike that got, or almost got, Buffy injured. Apparently, Buffy forgot about them as well, since she referred to Spike as "the only one watching her back."
The closest to any sort of "slip up' that might have gotten Buffy injured was in the basement when all of the freshly turned vampires came out of the ground and grabbed her, but she and Spike weren't fighting together then. In fact, Spike was under the control of the First Evil at that time, so I'm not sure that would count as a "slip up" on his part.
And as for the "huge crush" that Spike has on Buffy, the only time that it was a real issue was when he returned from his road trip with Andrew and discovered that he was the only one who still considered Buffy to be the leader. The result of that was that, according to Buffy, he was what gave her the courage to retrieve the Scythe and continue her battle.
None of that seems to be any sort of liability to Buffy as far as I can see.