I can see the need for 5 units. I've got 6 in my boat...one never gets used. Flush mounted 5". That said....

Console-
side scan unit
mapping unit (networked)
Purpose to be able to cruise a contour or feature on the maps and side scan. Or to come back by a way point already on the map, and scan it while the map is up.

Bow-
Mapping unit networked to console units. Used soley for mapping.

(On my boat):

12" Garmin for forward mode livescope
10" garmin for perspective mode garmin.

Not sold on perspective yet, for MOST of what I do, but can see it being super helpful in certain situations.

Can also see the want/need for another scope unit for a partner. It's tough to effectively and efficiently fish two people and one scope.