Raine - this is a very good question. RL's answer is an excellent one.
For me, also, a key component in explaining that something out of the ordinary is happening to our spouse/SO is the total change in behaviour not only towards us (which could arguably be caused by a marriage breakdown, in an very extreme form) but also towards other family members, children, old friends and even pets. The fact that they have effectively become another person.
Ultimately though the work of the IC is to help us deal with what has happened - but I do think that we need someone who 'gets' MLC. The first person I saw didn't, and did me more harm than good, as she insisted on dealing with it as marriage breakdown. I kept trying to say that there was more than this going on, and she would smile in a what I felt was a superior way and take no notice.
The second one did get MLC, although he put it a little differently (and that in itself was helpful) but he was in no doubt that there was something very very wrong with my xh. He saw his role as helping me to go on living my life, healing and dealing with xh as and when necessary.