Bones-
It will protect from the elements better because you can't get a tear in it or a pinhole leak. Tree branches won't tear/punch holes in a fiberglass one, straps won't come loose, velcro loosen, snaps come undone, etc. The air pressure going over the top of a rig lifts a bikini upward, at least it does on a jeep, whereas a fiberglass unit will stay put. Bikini's never seem to be the right size, not wide enough, etc. (I always get dripped on) so building your own would make it the perfect size. The most important part of it that you seem to have missed is that it is custom! Custom stuff (at least well done custom stuff) is cool. Anyone can buy a bikini. Boooooooooring. I never said that making a fiber would be cheaper.
EZ