People have and I've seen it used on exteriors with great effect.Why not? People have covered their whole cars with the stuff. It's also an easy way to salvage my splitter for £20 without buying a new one.
However... it isn't designed for exterior use so for an area such as a front splitter or bumper which gets a lot of stone chips I think it would look rubbish very quickly.
Unlike normal exterior vinyl it doesn't have that slightly rubbery feel or texture to it, which is part of the reason that it looks so good but also means that it is more likely to tear when being impacted by stones.
Just my opinion.