I don't use FCP but I do use BCC7 with Vegas Pro and After Effects which if I make your question more generic, it is fundamentally asking, "Do I need BCC7 in my NLE as well as Special FX application?".
I can tell you that I use After Effects a lot less now that I can do BCC in Vegas Pro. For me this is an incredible productivity improvement because I can create things right in Vegas Pro that I would have had to go out to After Effects to do before.
Having said that, there are times when I do need to move over to After Effects and for those times I want BCC there as well. So I need both.
If you told me that I could only have one, I would keep BCC7 for Vegas Pro because of the productivity boost of having 150+ new FX plug-ins right in my NLE. Only you can determine if its worthwhile for you to have both depending on how much you use AE and how much more productive you would be staying right in your NLE.