The fact that anyone in the Majors "can" throw the first pitch of the game, would seem to allow for anybody to start a game.
What I have a problem with is the fact that PB ALWAYS gives the pitcher starting the game all those extra BFA's. It would seem to me that if a pitcher has NEVER started a game, that his stamina simply would never allow him to face that many batters. Someone who HAS started a game is presumably capable of facing a respectable number of batters, since any MLB manager with half a brain would never throw out Mariano Rivera (Or Kerry Lightenberg for that matter) to start, no matter how desperate the situation.
A very simple fix is this : If a pitcher has never started in the ML, then he gets NO EXTRA BFA's.
If he survives 4 innings without getting beat up or injured - GREAT JOB, COACH !
I don't agree with inflating the BFA's of a pitcher who in real-life wouldn't be capable of making it that many innings...

As far as doing it in PB ? Well, the rules say you can. In my personal opinion, it is bush-league to do so, but if it is stated in the rules that it is possible, we cannot get too fired up if somebody chooses to do it....