...Well, you just obliterated my 'because it's a temporary RP' reason, so lets move on XD
What I'm trying to say is, even if there's a time limit, most temporary RPs want to get their members and start RPing. If it's on a noticeboard, there's a fairly large chance that people will start to join halfway through the RP (I'm failing at explaining myself again >.>). I'm not sure how to go about explaining it, but with the noticeboard you kind of loose the immediate recruiting that you'd get in a chat. Not to mention it'd take a while to find an RP you wanted... Unless they where sorted into different categories (Canine, feline, temporary, permanent, ect...).