i guess it is a good idea because with a huge slab of the stuff on your roof it moves the vehicles centre of gravity therefore making it top heavy...i guess it is aimed at 4 x 4 as they dont always far very well in the elk test in their standard guise let alone with a slab of snow on the roof 
Indeed TAM, it is amazing how many 4x4s I have seen on recent news clips about the weather that are on the sides or roofs. Too many of their drivers think they are invincible when driving those, and the inevitable happens!

Did you see the 4x4 on the BBC stuck across the country lane? One front wheel up one bank, one rear wheel up the other, at a 30* angle up on on end... I couldn't figure out how they got it
so wrong..
And then I drove to work this morning (and home) and noticed the number of drivers who's only attempt at clearing a window was to turn the bl**dy windscreen wipers on. FFS.
I've been out on the piss all night and I could do a better job!