Hi Mk1_Oz,
Water wont hurt a rose joint, its the dust and dirt that usually comes with the water that is the problem. In saying that, I ran open rose jointed arms in the front of my car on the road and track for 10 years and the rose joints are still ok. Bear in mind there are several different levels of quality available in rose joints, the quality is usually indicated by the lb load rating.
The benefit is increased feedback from the road/car and a very positive location of the component that it is mounting.
The disadvantages are a more harsh ride and more body shell wear an tear due to the harsh ride and increased feedback or shock transfer.
Cheers,
Phil.