It depends what sort of riding you do or want to do, and whether for example you feel the need for a singlespeed or something similar. Personally I'm happy enough to ride a geared bike off-road (I also ride other things which make singlespeeds seem easy), and whilst it might be fun to have a longer travel bike I doubt I'd want to ride the sort of terrain where I'd really notice the benefit over my XC full-sus (though I'm good enough to ride any trail center red route I've come across and also most black stuff on that). Hence I'm struggling to think of a situation where I'd really prefer to be on something different.
What differences do other people have between their bikes that make it worth having multiple ones for different things? Are we just talking XC/DH/jump here, or do people have multiple different "freeride" bikes?
Of course like njee I'm just talking mountain bike here - I also have multiple road bikes, and several other wheeled contraptions.