Parsers are in fact against the rules. However, SE treats them as a "see no evil, hear no evil" thing. If you don't mention in game that you have a parser, they don't care if you have one. If you do mention it and someone reports you, expect to hear from a GM. Yes, this does mean you can't kick someone and tell them it's because they have low DPS, but you can kick someone for no reason whatsoever ("different play styles").