The problem with this approach is that it means people can't build tools to run on their network (Thinkbot, for example), so people will be drawn away to other open networks like Jabber.
Microsoft Corp. is making changes to its MSN instant messaging (IM) service that will lock out users of third-party software that uses the service as well as users of older versions of Microsoft's own Messenger client
Update: Trillian 2.0 and Miranda should still be fine.
[Microsoft] are not 'blocking third party clients,' they're merely removing the expensive legacy support for really old-and-outdated versions of the MSN protocol to simplify their servers; any third-party client is still welcome to connect as long as they're using the more recent versions of the protocol.