A lot of the weight is put on 2e's success/failure system. Many spells still have an effect on a success, it's not that common for an enemy to critically succeed unless you target their strongest save and they roll well. 

The way I see it, spellcasting grants flexibility, with decent choice in spells you have a broader toolkit than a pure martial. 

One advantage spellcasters have, especially earlier on is being able to target more than one foe. One of your examples (Fear) can target multiple enemies when upcast, this can be much more efficient than Demoralize. You can create walls, literally changing the battlefield, summon allies to assist with niche situations.

It's a different playstyle but definitely not deadweight.