I would NOT like to see all those maps available in Navigation, since there is no purpose for it. Next users need only the best map. Other maps are only there for compatibility with other devices. They are there for planning purposes, not for navigation purposes. Just use the Here map for planning, and the compare function if you want to make your route equal for other users.
Is HERE the best map? In all situations? I don't think so. For example, (not) routing across ferries is a major weakness of the HERE map/routing algorithm. In addition, the free OSM map can be better in certain places in the world, as it can be maintained by local people, which can result in much better map data in places where the commercial map providers don't have great coverage.
And the other maps not being for navigation purposes is pure nonsense. Plenty of people navigate using TomTom or OSM. You say so yourself: "for compatibility with other devices". The entire world doesn't and isn't going to use MRA Navigation (Next).
Note: I fully expect MRA Navigation Next to only ever provide the HERE map for navigation, because supporting 3 different maps is a lot more work (not to mention it being more expensive in terms of licensing). However, it would still be nice to have it, so you can use your favourite map/routing algorithm and/or the 'best' map for a specific trip.