Aesthetically, it does look shit when any ground has empty seats. When a stadium is packed it does look much better.
And it is shit when games are "sold out" (I actually don't know if yesterday was, but generally speaking...) only for loads of seats to remain empty.
But both Milan clubs average around 35000 for league games, in an 80000 stadium; Barça have, on average, about 35000 empty seats for league games; United have lied about attendance figures for years; and last year I was reading on an Arsenal forum their fans saying they reckon their real average attendance would be around 54000 but they just 'sell' it as 60000.