Although I can empathise with the "common Sense" argument, there are practicalities to the "Crew sign everywhere" approach.
As you rightly suggested traincrew have to sign all routes that they work over, and similarly have to retain knowledge of those routes. If all drivers / guards signed all routes the workload would be spread so thin that route retention would be impossible. So a balance of workload is inevitable!
That said, whilst i don't think harmonisation is an easy task, i believe First are committed to it, and are indeed still trying, as we've got posters in the depots saying as much!
Personally I feel that with the recruitment drive undertaken, and the talks (maybe) progressing, there may be more flexibility in the future?
The same issue applies to traction knowledge. Even under
BR▸ not all links at every depot signed
HST▸ traction. The drivers were placed in 'Links' or base rosters. The Links were based progessively on seniority and it was only the higher links that signed HST's. Certain more junior links also signed them but not the routes to Paddington, the basis for this was that the more junior staff worked HST's to / from maintainance depots at night, therefore nightwork was the principle junior drivers workload.
With the seperation of GWT from
W&W▸ , the drivers were split between the two companies on a choice basis, but where this was not possible seniority was used instead. Therefore the nascent GWT, later
FGW▸ got most of the older drivers, who have in turn been repaced over time. The GWT / FGW links also have to cover their own night shifts, as there is no possibility of using 'West' men who very properly removed HST knowledge from their cards at the time the depots were split in 1994. And are now on a lower basic rate of pay. Several older drivers in fact opted out of HST work at the time of the depot splits due to the amount of nightwork that was required to be covered by the GWT / HST links.
Units however have the eminent and considerable advantage that they can be coupled together and taken into the depots in long formations with only one driver, therefore much less nightwork is required.
Quite a bit to sort out there I think!