I'm not too sure if I like the idea of trading pound for pound because some calibers have a much higher value than others. If we were only trading stuff like 9mm and 40 S&W then it might be a good idea. I'm a little too OCD about some things and counting brass is one of them.
EDIT: However, none of that is to say weighing brass to determine the approximate count is a bad idea. I did that to the 45 acp and 380 acp brass I have that's listed. I just don't think trading based 1 lb of brass for 1 lb of brass will always be fair.