Well having LR play an image split across two screens is very annoying, so quite a useful function I'd say. And why would you want to see whatever is on the other screen when presenting a slideshow? So blanking out the other screen makes perfect sense to me.
But I'm curious as to how you even do that as LR doesn't support dual monitors and you just need to see how many people as peed off with that on the Adobe LR Feature requests, possibly the longest thread there.
LR plays a slideshow split across both my monitors and I've not been able to find any way of playing a SS on just the single monitor, which is what I would prefer.
Have you got LR 2.0?
BTW when you export as PDF, as opposed to just playing a SS, you get the option of the PDF playing full screen.