The leisure=stadium tag implies building=yes/stadium/multi_storey/.... Otherwise, the thing is a pitch.
Don't believe me? The Memorial Coliseum and the Rose Garden Arena are both stadiums. As of 12-mar-2010, someone has decided that the Rose Garden Arena also needs an explicit building= tag. See the difference yourself.
Interestingly, there are some facilities that need the building= tag to render. For example, a bus station renders but a train station won't. Compare Union Station to Grayhound. (Well, or you could but the railway=station tag has been removed from Union Station, sigh.)