You'd be surprised at how lazy people are about checking their files. I obviously block the typical T1/T3 servers that spout out looped / beepy versions which increases the quality of my searches. But when I browse ordinary users I often find they have a mix of perfectly good files and corrupt ones or demos. In these cases, there is clearly no malicious intent it's just that they don't check their files - or don't care - and there are too many of them to block, apart from the fact that they may also have good files that interest you. All of the cases that have recently offended my ears have been this type of user.