I can see both sides of the point of not having a line at the lavs. Maybe it isn't a security issue afterall.

On one hand, when you have to go, you have to go. Not only is it not easy to run down an airplane aisle, it would also probably cause some attention. Definitely better to be able to stand near the lav and wait your turn.

On the other hand, how long of a line is too long? Two people, four people, a half dozen? Look at from the passenger who is sitting in the aisle seats near the lav point of view. When you are sitting, you are basically at rear-end level. Do you you want someone's butt in your face continually throughout your flight? Especially if they are having bathroom issues. Also, there is the hair pulling. Not that this is done intentionally, but people seem to place their hands on seat tops which inadvertently causes hair pulling, which is also something that doesn't make for a very pleasant flight.