I believe that it is nothing to do with the prospect that engines might flame out during taxi, or even during normal takeoff or go around. Any engine that was inclined to do that would be unlikely to gain certification. Early CFM installations did suffer flame outs and roll back from water ingestion but that was fixed 20 years ago.
The requirement to use ignition on takeoff and approach probably only remains as an additional layer of protection against compressor stall, water ingestion or bird strike.
The reason for moving ignition from when it was originally done at line up to the after start checks came in when Boeing rightly decided to cut down checklists when the aircraft was on the move. Boeing always was nicely brief, but now their taxi checks are absolutely pared to the bone.
But good point, selecting igniters on descent would continue the philosophy of reducing the reading of 'stuff' in checklists at busy times. Maybe it should be tied in with resetting altimeters passing transition?