Three piece, five piece whatever. If you're manufacturing hundreds of guitars a shift it allows you to use sizes of timber that maximise how many necks you're gonna get out of every cubic foot of timber you buy and make a neck from timber that would possibly distort if the the neck were in one piece of it. That's why the big manufacturers took up that option; not to give you guys a better neck. For instance, despite the propaganda from some of them, a scarf joint between the second fret and the nut is not going to help your tone one little bit.
Having said that; intelligently mixing materials in the neck can let you really nail the tone you are looking for and give you something that is extremely stable. I really like using complex composite neck structures and have used it to great advantage on a number of guitars, some of which are owned by contributors to this forum. It makes it possible to combine the attributes if different types of tonewoods in one structure.
So I guess if you're looking at a composite neck you have to consider A) what is it made of and B) are the joints there to give a better tonal response or to utilize scr@p pieces of timber