Tools they could add besides DPS meters... (For endgame)

[Vahlnir]

It's not against the rules to use a parser. If you use it to harass someone it is, but otherwise you can use them without a problem.

It is against the ToS (officially). Unofficially, the devs don't care so long as it isn't used to harass others and players keep quiet about its usage. If you were to, say, post a parse log into a party chat with random players who may or may not be okay with it, you could be reported and there would be evidence for action against your account. It's more of a "don't ask, don't tell" policy.