Some software does use it, eg. I think Worm Wars and AmiArcadia do something useful with it IIRC.
Personally I have the MMB set up to simulate doubleclicking the LMB, in both AmigaOS and Windows. It makes those OSes much more ergonomic. Doubleclicking is one of the most poorly thought out UI elements, whoever came up with the idea should be shot. Probably some lamer from Apple or Xerox.
