I think Webber was hyped by his rather powerful manager, Flavio Briatore, in the early to mid 2000s in an era when the driving depth at the sharp end was not as strong as now. Since then Hamilton, Kubica and Vettel came along and replaced Montoya, Villeneuve and Coulthard respectively.
So whereas Webber was a 'qually specialist' vs lesser opponents and as a result was 'delivering' to some extent, the quality of opponent is higher now (in my opinion).
It's said - and agreed - in this thread that Webber was a 'world beater' for about 1 week or 8 or 9 days in 2010. Sadly it's also known that Vettel's chassis had a slight crack in it in those races (Spain, Monaco) among other problems.
In any driver pairing someone will lose. That is the nature of things. Before 2010, Webber was hugely out-qualified by Vettel and it's been that way in 2011. Funnily enough, Webber was closer in qually to Vettel last year than any other year. Perhaps 2010 was Mark's peak?
In the end, Webber is a decent Number 2 but probably not in the Top 5 (or Top 6 if you include Kubica at his pre-accident level.)