Voidmage Apprentice is a common creature from Legions that also cost 1U to cast, but also was only a 1/1. However, for 2UU (2 colorless, 2 Blue), he could morph to counter any target spell. That's a total investment of 5UU to target a spell, which is pretty good in Limited (especially at common). Stratus Dancer is more like a limited Negate, but with only a 2UU investment.
Kheru Spellsnatcher is an interesting card, but its Morph cost is 4UU. Then again, you can steal the spell you counter with the Spellsnatcher's ability. It's been fun to play in Khans Limited, but Stratus Dancer is a lot cheaper.
Being a flyer is already a plus for the Dancer in Limited. Also, the Megamorph ability is a fine way to counter removal. Since removal is at such a premium in Limited, the value of having a 3/2 flyer that can counter many kinds of said removal is huge. While I don't see her as much of a Constructed card, Stratus Dancer is pretty solid for Limited. Were that Megamorph ability expanded to include all non-creature spells, even if it cost another mana, then I would see it being played in some sideboards.