It would be fine for me, but ONLY as far as it would be promised to be compatible with all future MSN Messenger builds (at least MSN Messenger, if you decide to *forsake* Windows Messenger), and be promised to give plug-ins full access to Messenger interface...
*
Personally, I still want to see some more to be added to Plus! such as rich-text-format chat logs and built-in Personal Message logging... but softwares have always got their limitations, so I think I'll be able to live with it.
Don't worry, Patchou, that you will lose user support... Microsoft, I believe, is getting new Instant Messaging users every day, and there is no doubt when 100 people download MSN Messenger, 99 download Messenger Plus! too (the rest one is mysteriously murdered by Avada Kedavra after he installs MSN Messenger). The number of downloads you have in your statistics is growing and that explains everything, innit?
* That means you can never drop Plus! when MSN Messenger is still gaining popularity