Showing my ignorance one again!

Assuming you have a preamp that will supply a -12db signal slope, both 'fore' and 'aft', for any driver with a 'sweet spot' within that range, and an unlimited number of amps for each associated preamp out, is there any reason for a crossover? No matter a 3-way, 4-way, etc.?

Educate me, please,

Greg