Dan,
That might be true to some extent, but it could still be useful for a few reasons:
1) if you have to delete 400 emails asking you to support something - even if you don't read them, the sheer volume might affect your conviction to oppose it
2) if you have something positive and brief (and coherent) the reader may at least skim it for important messages before deleting
3) 1 in 10 might read it - I have no idea who that might be, so I offer everyone the opportunity of receiving it.
I never hide that I'm out of their District - infact, we lavish the praises of our local legislators and how proud we we are of them and their stands on issues and their votes. Tell them how thoughful they appear even to people from outside their district, and how their support on the issue is both important and noticed. I don't think it can hurt to appeal to them this way. Probably doesn't work, but I sure don't want to be left thinking I could have tried harder.
M.