In my limited exposure, I can't envision many environments where the aircraft is the tallest object around. Even at the most desolate unimproved desert strips around here, there was at least a saguaro that would more likely attract the lightning.
That said, the aircraft themselves are probably pretty well insolated from the ground, and without doing all of the math, I would imagine the path of least resistance for the lightning is not generally going to be through the aircraft...even in an open field...but I could be wrong on that.
On the flip side, grounding the aircraft is probably going to do more in the way to promote electrical conductivity to the ground, and that would likely have negative affects in the way of corrosions.
Now...all of that said, my last point makes everything I said above moot. When you tie down your airplane with chains, and even rope, you are doing a pretty good job of grounding your aircraft anyway. Rope may not be ideal...or even good in some cases, but probably sufficient.
What ever the real answer is, I highly doubt that adding another thing to the ramp area or any tie down that can potentially get wrapped up in a prop, or neglected in the pre-flight is worth the effort and potential problems.
Just my $0.02