I think this is the point. It is true that the commander doesn't "keep up" with units he leads using movement enhancing commander promotions. But you're making the assumption here, that all movement is in a straight line from A to B
The purpose of recon units, is... reconaissance. With promotions to enhance their movement, a recon unit can break off from the army, to climb a nearby hill. and get a view of the area, enemy troop layout, etc. Then still have enough movement to rejoin the army and continue marching with the melee units.
Or they can break away to intercept an enemy settler/worker that's exposed. Or get a quick attack of opportunity on enemy reinforcements that are passing through a desert.
If you think of the movement in a straight line, then yes, a commander is not going to keep up with a 6 movement hunter. But, if you treat the commander and his army of melee/archery units as a mobile base, you can use said hunter's 6 movement as an operational radius, allowing you to launch quick strikes out at vulnerable targets or just recon the area, and then return to the safety of the big archer-protected stack in the same turn, never getting left exposed.
Command range promotions are indeed your friend.