You do legally need it, it's a part of the emission control system. There is a valve in the intake that is open when the engine is cold to allow hot recirculated air to be drawn into the intake. The valve closes once the engine is warmed up so you just take in cold air. Something to do with making the engine not run as rich when it's cold. Multi-point engines don't have this though, for reasons I still can't quite figure out. Probably because fuel delivery is much more controlled with multi-point.
You won't see much of a performace gain with a mushroom filter thingy anyway, as it's gonna be sucking hot air once the engine warms up regardless.
Converting to multi-point is a far better option. I'll be doing mine soon.