[Ashenlock]

All true, but some players will run "veiled" Trick Room users like Chandelure and Jellicent, that can function in either setting, and that have significant offensive abilities. There isn't much difficulty in doing something or other to stop it if you are 99% sure that the opponent is running Trick Room (relatively speaking, at least), but there is if you're not even sure that you won't be wasting a turn using Taunt, Ghost Gem Shadow Ball, Imprison, etc. And it happens that many Pokemon that have that awkward Spe stat between Trick Room-level and sweeper-level also have great attacking stats. What Cresselia lacks in offense she makes up by having that ability to conceal her purpose.

It's not that I think it's bad, I just think it's not much more than an above-average means to stopping Trick Room. Two of your team's twenty-four moves are devoted solely to stopping Trick Room, and don't do too much more than that.