I believe they decided to do this not bcs of their "obsession with surface effects" ... but more like "bcs Extended Spell would otherwise do litteraly nothing in most cases" :-/
Extend spell are meant to prolong spells that has a duration of 1 minute or longer. For example buffs like haste, invisibilty etc or debuffs like slow and fear. Those are really powerful spells so doing nothing ain't exactly how I see it
It is not meant to be used for short 3-4 turns debuffs, and in RAW it doesn't.
Now, imagine multiclassing a sorc with cleric, getting access to divine buffs and extend spell definitively gets worth a sorc point.
