There's two ways you can look at it. One is that our current "i686" target is buggy because it doesn't actually work on all i686-class machines. The other way of looking at it is that using target triples to specify CPU features is a bad idea because we try to attach too many different meanings to the triple, so the right solution is not to add an "i586" triple, but rather add some other mechanism to distinguish CPU features.
Either way, adding an "i586" target to rustc without any other changes would be extremely confusing.