For one thing, it's illegal by international copyright laws to posess pre-release software, beta versions, or otherwise illegaly obtained (downloaded) software if permisison is not given by the producer to have a copy.
Secondly, it's harmful to VALVe and other software companies when people download leaked betas; These people often won't buy the game at all, or just download another copy when it goes final. This means that those 1000 people won't mean profit for VALVe even though they have their game, which, if enough people pirate software, can drive smaller companies like VALVe out of business, and leave us with incredibly rich software conglomarates that pour out crap games. Most of the better games today are produced by smaller studios. If the studio does not profit from their game (usually only one in production to the size of the studio), they cannot pay their workers, and thus most of them leave and the studio goes out of business.
Think of it this way. If everyone pirated HL, then VALVe wouldn't have profited as much and wouldn't have been able to put as much effort into making the great game that is HL2, as staff cuts or less development budget would affect the game.