Speaking personally, I wish some of these acts would just be honest, and say "sorry guys, the numbers aren't there".
Last time around it was "visa" issues. Before with Pain of Salvation it was "we hate Bush and are protesting."
Maybe it would be nice to hear "we're largely a studio band, we thought things would be different 8 months ago, but we've failed to secure other dates around this time that would make getting together, rehearsing, and flying across the pond for this show worth it."