The problem is, once such an online service is well known, the hackers could just blacklist the host name(s) to the service so no alerting messages are sent.Agreed, this is another problem. However, I think that it also depends on who you think might be targeting you. I know that a few years ago, plenty of everyday non-technical folks "hacked" their ex-girlfriend or boyfriends' accounts by doing a password reset with known details.
If the aim is to get notified of similar issues (e.g. my home laptop accessing during office hours), such a service could have merit.