You know what was FAR worse than any parser in WoW? gearscore! Which we already have in the game with avg ilvl and that only gives you an idea of roughly how much dps they should be doing and allows people to discriminate based on a number that doesn't actually mean much.

Parsers allow you to get meaningful data and then use that data in meaningful ways that benefit the group, not just dps but tanks and healers too. You can check what hit the tank, when it hit and for how much damage. You can then check if a CD was used before, if not did they have one off CD? if one was on CD can they save it for that specific ability or we we need to ajuust the healing to maybe apply a stoneskin/adlo just before it comes?

You can do this without but it takes FAR longer to piece this all together when at the click of a button you can just check.

I played WoW for 6 years and in all that time the amount of harassment due to parsers was negligible(people don't need parsers to be a douche), in fact the use of parsers allowed many people(including myself) who were massively under performing to be either on par or exceed what is expected of them.

That brings me to group roles, as alot of people have already said it's pretty easy to point fingers at tanks and/or healers but trying to pinpoint which of the dps isn't pulling their weight isn't an easy task. As a dps player my role in a party is to deal as much damage to the opponents as possible while also dealing with the mechanics, if i'm not doing that i'm a hindrance to 7 other people. Now spin that round to the role of a healer they are charged with keeping people alive while dealing with the mechanics just like everybody else, if people are dealing with mechanics and yet still dying the healer/s are at fault and they are a hindrance to 6/7 other people if they don't attempt to fix it. Right now there's thousands of players that could(and do) see immediate benefits from having access to parse data and then there's thousands of lazy players that are able to hide that they can't be bothered to learn their class and quite frankly I don't want to play with a lazy player and before anyone calls me an elitist thinking i'm above the "casual filth" i've sat for hours and coached people who wanted to learn and improve, people who just watched me hitting a training dummy and without directly saying they were using a parser said something like "I saw how much damage you did", they wanted to know how they could improve their damage to be more in line with the gear they have and I have time for those people who want to learn and i'm always thanked for it.