Right...if Data could lock them out the Bynars could have done the same (which, granted, they could have brought up in the episode). My point is, in this episode Riker and Picard are on their own, while in "Brothers" the ship is fully crewed and dealing with a medical emergency. It's not the same scenario, in that if Riker and Picard separate the ship, then sure the saucer's dropped to impulse, but it's still just the two of them..at least while the saucer's attached they can access the bridge physically (or, as shown in the episode, beam onto it). So, beyond dropping out of warp, what would a saucer separation actually accomplish?