If the TV doesn't capture it then it leads to nothing important. Besides, when I was a match goer I spent the vast majority of my time watching the ball (and cursing...)
I think in the context of a conversation like this it's relevant. If a full back is consistently out of position/making poor choices/not showing for the ball you don't really see it on TV but it's obviously a problem. The thing is on the occasion that it then does lead to something, if you've only ever seen him on TV you're maybe likely to say 'oh that's a rare mistake from him' whereas if you're watching him live you know that he does it all the time.
For me all the talk about last season just misses the point that he wasn't actually particularly good then either. He was one of the better performers for a team that was very bad for huge portions of the season, but that doesn't make him good in real terms. I think his level has dipped this year, but that's the problem when your best level is bang average, any loss of form is a huge issue because it takes you into unacceptable territory.