The IMS and related IETF specifications define a long list of ingredients that can be used to make a lot of meals (from the pizza to the gourmet menu). The only problem is that so far nobody has delivered any cookbook for it.
Once IMS has been deployed the question can shift from "do we really need IMS?" to "how can we make an optimal use of this f*** IMS we spent so much f*** money on?".
PS: I have a few slides on this subject, that I had the opportunity to present in a conference. Feel free to email me to get them.