Well I'm sure the idea is to make it harder to turn off the unit when playing a game. If you want to watch a movie you need to eject the PAC anyway, as with a PAC inserted it only outputs in 240p, as opposed to the 480i that most LD's are recorded in.
Personally I don't really care about my missing PAC cover. To me the LA is strictly a gaming device, if you want to watch movies, you can get a better player for cheaper anyway.