Once you get to about 1 meg it's pretty much like having no pot in there at all when it's full up. The resistance is more of a ceiling. Since guitar pickups don't put out that much signal, 500K pots are fine in almost all cases. You could argue that a 1 meg pot is too much room and that it makes the guitar screechy. There's a smoothness to aptly matching the pot to the pickup's output.
So a 2 meg pot isn't really going to be any different than a 1 meg because the pickups aren't hot enough to be affected by the extra headroom. So then the next thing is the taper of the pot. With the extra headroom of the pot, the beginning travel would be less effective and the lower part more effective. I wouldn't like that.