You're making the mistake of trying to judge a player on his "top level". When he's at his absolute best. That is not completely irrelevant, obviously, but wildly misleading. I mean, David Bentley once scored a hat-trick against Manchester United. Hell, James Beattie had a season when he kept up with Van Nistelrooy and Henry on the scoring charts.
If a player is shite most of the time but every couple of years he has a purple patch of a few weeks, then that player is shite. Being able to play at your best most of the time is an important quality for a footballer and cannot be ignored. Saying that "if only he performed like that more often, he'd be great" is as pointless as saying that "if only he were faster, he could compete in the Olympics".
Or, to use another example: I generally suck at online shooters. Sometimes I put together a great kill streak. Does that mean I'm good enough but I'm usually not trying? No: it shows that I can occasionally punch above my weight despite being shite.