The problem is that if you put an additional crewmember onto a 2 man flight deck just to operate a 'new' piece of kit, for 99% of the time he will have NOTHING to do. As that would seem to be a waste of a presumeably highly qualified aircrew person, the temptation would be to give them other jobs, just to fill their time up, such as the radios or reading the FRCs or even God forbid, doing navigation, & before you know it you have a 3 man flight deck because you can't even go to the USA without that person because you've built them into the system.
Or the WSOp(ALM) could do it, with no increase in crew numbers.