While I am not a big fan of plugins in the Lync client it seems as more and more traditional voice providers develop such plugins. To be a bit cynical, I believe that these organizations are trying to "help" their customers to avoid the costly Lync voice licenses (Lync Plus CAL) and instead get voice and such licenses from them. It could be a valid reason for an organization sitting with an existing voice platform wanting to wait before going all-in into Lync or wanting to save a few $.
When it comes to functionality these plugin solutions always becomes a bit messy.
Seriously, would you like to install a little plugin phone into an existing deskphone so that you could... call? or what about a Rebtel plugin into Skype?
Anyhow, here is some new stuff from Avaya:
Avaya Agile Communication Environment (ACE) version 3.0 for Microsoft Lync
that will be out soon, and here is a new blog from VOX covering a lot of Avaya stuff.
It will be interesting to see how popular this Avaya plugin product might become further on.
So far, I only had some experiences with Cisco's Cuci-Moc and Cuci-Lync plugin products, and I do agree with the opinions of Luis Ramos and Justin Morris.