They can keep Skype for whatever it is they use it for, and use MiCollab as well for its softphone, twinning, integration to the phone system and corporate directory.
If you go with just Skype you can add the Mitel Plugin to it so that Skype can send calls via the phone system, you just have to do whatever it is in AD so that Skype has access to the users' phone numbers, seeing as Skyps has no other way to access a corporate phone directory.
If you want a seamless phone system integration and softphone functionality, go with the full MiCollab Client, otherwise go with the sparsely-featured Skype option. The latter is pretty bad, which is why Microsoft are ditching Skype, although the replacement will no doubt be similarly crippled integration-wise in order to try to convince you to replace your phone system with whatever Microsoft offers.
