My reasons to be against parsers (based on experiences in other games):

1 - I don't want to be under the watch of strangers, posting in chat the raid dps after each trash pull, like it already happens.
2 - If without much information on the screen players already mess up mechanics, imagine if they go all "omg omg omg, must keep my DPS high!". (this one already happens, too!)
3 - If i wanna perfection, I stay with my static only. If I PuG then I have to expect more of a random skill lvl from the random players that will join my group. If I start policing everybody's performance I will start joining more blacklists than RMT's.

PS: if you don't wanna PuGs to "waste your time", don't PuG.
PPS: Many ppl are openly admitting to use parsers in this thread, I suggest you guys to edit your posts. :P