Entirely possible to never swap out of MW as far as getting dailies done. Does very respectable damage, especially over relatively short time periods like normal mob kills.
I started out MoP levelling 1 - 90 on a Mistweaver. From lvl 14 up to Cata, I levelled exclusively via LFD (and the occasional SW fishing or cooking daily, lol); by the time I hit Cata, LFD queues started getting longer and I started questing while waiting for them.
Your dps *does* start to lose pace with mob health pools in Cata (ie, you aren't necessarily killing a solo mob terribly quickly) but you are virtually unkillable, and once you're comfy with accessing your utilities for group pulls (Fortifying Brew, Paralyze, Spear Hand Strike to interrupt casts you don't want to take to the face/pull casters into your AoE range, Life Cocoon, etc... ) you can start to pull *large* numbers of most mobs and kill them in virtually the same amount of time it'd take you to kill a couple.
This doesn't *always* make questing faster (... you can't always just gather up a group of mobs you need to AoE them down) but it's worth mentioning.
By the time I hit... 88? I think? it really *had* slowed down for me, and I grabbed a WW spec. what I *should* have done was grab a BM spec; I ended up enjoying BM much more than WW, and BM does an appreciable amount of dps even though it's not as much dps as a WW.... and I liked the survivability of the BM over having to survive as WW (any half-decent WW shouldn't have any trouble, but let's just say I wasn't even a half-decent WW ;D )
Like Juju says, above me... end-game with only a tiny bit of gear, your MW dps scales really well against mobs out in the world.