In our case BM refused to use it at one mediation after DH suggested it. I assume it was a control issue. Then at the next mediation, her lawyer suggested it (I think she probably did not realize he was suggesting this), so DH had his lawyer respond that of course, he agreed, because he had suggested it previously. 

I think for most HCBMs it's likely a control issue. BM in our case, will still regularly text, instead of going through OFW, but their agreement gives DH a good enough excuse to ignore her texts. He's regularly replied to her on OFW, even though she has sent messages initially through text. 

BMs also have to watch what they say on OFW since eveything is saved and there is no disputing it. Our BM tried to change the calendar. She claimed she put the new dates on the calendar "a year ago", but OFW timestamps everything, so it was clear she only changed them a month or so before, without notifying DH. We then put all the dates on the calendar and BM just schedules over them, but again, with OFW, it's easy enough to show that she just scheduled over without consulting with DH. 

Control and lack of interest in truly creating an official record of communications.. when she can do it on her own it is a lot harder to pin down.

Whether she refuses or now.. your DH can still set his own boundaries for communicating and refuse to answer random non-emergency texts.. and perhaps just doing it all via email for example...

There is no way to delete anything once posted.  She can then say , She did not say that, or agree to this. 

If OFW is COd, ignore her other contact attempts, keep posting on OFC, and submit a co tempt motion every time she fails to communicate through OFW.  A call, contempt motion, text... ditto, email.... ditto, snail mail.... ditto.