Swiftman, I have an Abbey Vogue GTS, with one piece grp sidewalls and the main advantage is that there are no joins anywhere to allow posible water ingress.They look really good too, a very smooth finish.
The only possible downside that I can think of is if you get any damage, say a small area damaged, then its a case of either replacing the whole panel, at I would guess a horrendous cost, or being GRP, having it repaired, which shoud be relatively easy, and a lot cheaper.