Just quickly returning to the thread...and my OP seems to have been totally ignored. This is about the increase in the numbers of 'Ninjas' or how you like to call them.
I can't answer this directly; all I can say is that there was definitely no shortage of ninjas when I ran FPs after launch (before LFG and F2P). Haven't really bothered with FPs since - but not because of ninjas.
I don't know if there's been a change in the frequency of ninja'ing. Just to hazard a guess, maybe the population had reached an unusual equilibrium point pre-F2P, and F2P just brought the game back to MMO reality? I dunno.