You have to wonder why someone who can't stay in tune for the four words 'give peace a chance' thinks she should be on stage with a microphone. It's Lennon's fault really though - why did he let her?
Anyway, I don't think there was ever any intention of Eric Clapton becoming lennon's permanent lead guitarist. It was obviously a 'pickup' arrangement, as Lennon explains to the crowd ('we're just doing songs we all know'). Other than that, Plastic Ono Band simply meant Lennon and whatever backing musicians.
This is worth seeing mainly for Lennon doing that teen shuffle left over from his Beatlemania days. I suppose it was instinct but, with his Methuselah beard and specs, it's quite a sight.