I guess the reason I would prefer no symbol is that I very much disagree with the position that "I'm an athiest" means "my religion is science". Most people attempt to form most of their conclusions about the world on the basis of observation and rational thought. They make an exception for some things, where they choose insted to have faith in what their religion teaches. I feel that I do my best not to make such an exception at all, rather than to have blind faith in science, so I feel that a lack of symbol is much more appropriate than a symbol of science.