2 piece rotors are made to address thermal expansion of the rotor. A full floating rotor allows for growth in the radial direction (look at the small gaps between the steel braking surface and the aluminum carrier) and also axial float. The axial float helps to minimize/eliminate rotor runout and pad knock (not such a big deal in mountain biking but in auto).

 I just can't see what's so good in having a 2piece rotor? Every single 2piece rotor I've seen had problems: it had slack after a while or it broke.....

 they disapate heat alot better and are usually way stiffer