No huge reason. The original ASIO support was a contribution, and it never worked with my soundcard until recently. There is another patch I have in my in-box to add recording, but it needs a few little tweaks before it can be merged in.
ASIO recording isn't separate from playback like other driver models - you must deal with both outgoing and ingoing buffers in the same callback. It's a model that I really like, but it does mean that it won't quite fit with the other IWaveIn implementations.