I'm not thinking of multiple extruders that are active at the same time - multiple controllers and motors, etc., would be expensive.
I'm thinking of multiple syringes, which one extruder could switch between. So you could only extrude one material at a time, but could swap between them. So it's slower, perhaps, but cheaper. Imagine, for example, being able to load a bunch of syringes into a rotating drum, so you could rotate the drum to swap materials. And as a plus, you could have all but one of them blocked so they won't drip!