So i have noticed a lot of bikes that have just one all carbon 3 or 5 spoke wheel in the front (sometimes in the back). Along with a full spoke wheel for the back. The question is, what is the point behind that? I have been able to come up with some ideas, but not sure what is right:

1. Back wheel takes more of a beating and needs to have a stronger spoke. (not sure if that makes sense considering carbon is pretty strong).
2. aerodynamics, are better. but having one in the back doesn't matter as much due to your legs causing drag before the back wheel anyway.
3. Just for looks.
4. all carbon wheels are expensive, so most people start with one in the front first.

combination of 2 and 4 seem to make sense to me.

Either way, i am just wondering if there is some performance benefit that i am not getting...