It's far more cost effective to extend a spell instead of casting it again. It just tends to be situational. 1 sorcery point for 20 minutes of clairvoyance is better than casting clairvoyance twice. The only point in taking that metamagic is to help conserve spell slots.
Someone who uses charms and dominates in town might make use of it puppet style. Long term buffs are usually the way it's applied.
Extend can still save spell slots. Clerics just add to the list of spells it works with. The 8 hour spells it works with just enable buffing for the next day and walking into the dungeon with a full load of spell slots. It's not like squishy party members would not appreciate mage armor or humans given some darkvision if the sorcerer buffed other party members with them before resting and also took those spells.
The actual number of spells known might get in the way.
How often it actually matters as opposed to recasting a spell is where it becomes questionable. Extended spell is not as decisive as other metamagic options available.
Aid seems to be the most attractive synergy.
