It's a matter of opaqueness, not of abuse: the install system will be unable to reason about what an arbitrary script does, severely limiting functionality. For example, pip will be unable to ensure dependencies remain constant (after installing something else, after DST begins or ends, between a dry run and an actual install, after unrelated software alters environment variables, etc.). This is much worse than a 95% correct dependencies manifest that can be easily hacked to 100% correct.