There's been instances, fleetcatches notably where alliances would definitely take advantage of this option with the players who know they can't be on to recall. Many big battles are decided by who has the balls to land, rather than anything else. I certainly don't think the alliance unable to cover the attack should have the option to send dissuasive defence without any ships physically available. It also ruins the idea of faking early on in the night, and structured well thought out attacks in wartimes.
On another note, the whole point of PAX, it's big advancement is that the game's far more aggressive, the emphasis is on attacking. This would limit attack success significantly. Actually, when you think about it..
You can fake defences anyway with the current system, harpies make great dragons and the attackers can't fleet scan or anything, if it's a real problem to ya just do that :\
We might as well throw in the towel if we allow it - or let people prelaunch attacks !
