Well, it would nullify her shroud to a degree...I would probably take a spirit visage against her in lane though. She's squishy enough to die in 1 or 2 combos anyway that I wouldn't need a sunfire or brut for damage.
the better question is why boots of swiftness lol --- I mean if there's people around its usually a good idea to go semi-deep into the woods so they can't blatantly see your wang, unless of course you are Myst2-LTJ
If they don't have any hard CC, why not? Shroud slows and swiftness reduces slow potency. Merc wouldn't help because you're constantly slowed while in shroud and instantly unslowed as soon as you leave it.
It's not very common for a whole team to have no/very little hard CC, but on those occasions where they ****** up and don't have any, I definitely get swiftness.