mccartney IS a great musician, he IS a great performer, and if you had any sense, youd realise that just because of his limited technical abilities with instruments, it doesnt mean he isnt also a great improviser.
I've mentioned McCartney before w.r.t a program he was in with Jools Holland about the piano.
I think it's called "The history of the jazz piano" [I'm not 100% sure because I missed the beginning, but Kemble mention a program Jools Holland visited their factory to make with that title]
Watch it if you get chance. It's a chance to see McCartney and others jamming and talking with Jools Holland, it's not the same as having Steve Vai sat next to you with a guitar, but it gives a better indication, imo, of his ability than what he chucks on record via a studio and producer.
Performance-wise as a member of the Beatles he probably rarely had to play, as they once remarked to Led Zepplin [who were doing 2-3 hour long gigs], they could stand for 30 minutes with girls screaming at them not even playing because no one could hear, they remarked they could get it down to 20 minutes if they were luckier. They certainly never did a "show" in the sense that Madonna might be considered a performer. He doesn't have the presence of a Robbie Williams or Freddie Mercury.
I think what he has is a "once a member of the beatles" lifetime card. and a few audience sing-a-longs that come with it. The guy who wrote Angels and so on for Robbie Williams, and those that wrote the Queen athems are in the same boat song-wise and I would argue put in a better performance from a showmanship pov too.
What I've seen recently [Like the gig MSN covered - possibly at the Cavern? and live aid / live8] weren't impressive. A catchy sing-a-long for the audience. Although some of musicians he gets to play with are.
He's written some catchy stuff, but as I said, I think so many have it's moot. Including his mate who got shot. afaict he's spent all the time since that day realising that getting shot was yet another thing John for the Beatles and his solo stuff that Paul never managed to beat.
IMO, in that program, he comes across as what he is, which is nothing particularly special. In terms of technique as you note he doesn't have any. In terms of improvisation he is not even close to the ability of the others featured in the program.
He's bluffing in the program and you won't need any great sense at all to see that. None of the other musicians were Hamelin or Horowitz, it's not about raw technique, but nevertheless they were muscians. He is a celebrity who was in a band created by Brian Epstein, with records produced by the beatles talent, George Martin, who correctly assessed the Beatles as "awful" when he signed them and has at least some of the characteristics you'd expect a modern day talent to have. i.e The Beatles had early Simon Cowell, Louis Walsh alikes with musically skilled producers hyping up an otherwise average band.