fibreglass with paint on is very hard to fit without cracking the paint, fibreglass can flex, paint cant, something has to give. have you tried picking a 2 mat bumper up without making it flex, it requires 3 people and patience. if you are very careful not to flex the fibreglass while fitting it you wont crack the paint, but it isnt easy at all, and then it usually cracks anyway when you tighten the mounting points, causing flex.
i agree the most critical stage is preparing the kit prior to painting, but you must be a god if you've never cracked the paint on fibreglass, with hands that steady you could be a ninja master.
with a plastic kit you can paint it, swing it about, hit people over the head, and the paint is still on perfect. you can whack it into place with a fist and not worry about it.
you can make a hat out of paper, i saw it on art attack, it even keeps off the rain, but only for 5 mins, and people will laugh, much the same as fibreglass bodykits