You should tell them that, with a pay-to-use board (such as the UBB) any security related issues are worked out pretty much immediately (at UBBCentral). Plus we have large communities (well, you're here aren't you?) for modifications and free support...
Besides, I woudln't be caught dead running a freeware forum (like YaBB) which are under constant scruteny (Ya know, a bunch of "presidential" sites ran YaBB Gold in its infancy, a large portion got their admin accounts deleted as soon as it was discovered their site was running a version of a forum that leaves all info public so long as you know where to look)...
