Some software is very specialisd and even if stable, in itself, it may have the capability of causing problems with other application-software and/or the OS and/or hardware. There are probably several packages that are highly stable but really not suitable for use by the uninitiated. It could be argued that such packages should remain in the testing repository so as to ward off unsuspecting users, while the package is also given a rating on the basis of its stability/bugginess?.