the value in the vivaldi project is not the plasma active software, its the FOSS friendly hardware. Once you have that you can experiment with whatever software stack you like.
There are various reasons to want a traditional linux based stack. It means you can run any of the 1000s of existing linux programs. (For example I used to run gnumeric on a nokia 770 tablet running maemo (the predecessor of mer)).
It is important though to ship new hardware with some nice software. openmoko got some bad press for stability and usefulness of their software, when the neo and freerunner came out.