They let them see and film it, whatever "it" was. Fast as lightning, that's for sure.
It is definately in the military's interest, to perpetuate the belief that they are working on or with projects so secret they may be "indistinguishable from magic" to the average layman or, more importantly, to the average enemy of the state.
Area 51 is a gold mine of mis- and dis-information for the military. By keeping enemy's guessing, you are always one step ahead.
It's no different from refusing to announce your starting quarterback when approaching a big game...or claiming he has been injured and may not play as well. Professional soccer teams are masters at this tactic.
Keep 'em guessing with smoke and mirrors is what it's all about.