Originally Posted by
Eriol
Not necessarily IMO. Some of the earlier answers (like Sickening Radiance) also offer another path: burn through the resistance. In many ways, Phantasmal force is almost as good as a recurring-save spell like Radiance, in that it requires a save, that they either burn a legendary on, OR they have bad stuff happen which requires them to use their ACTION to break through (not a legendary resistance, thus you're turn-denying them too), and all for a level 2 slot. That's pretty nice.
So I wouldn't discount the utility of using "decent but not horrific" low-level spells to burn through a big bad's legendary resistances. Just bad enough to get the DM to consider burning the legendaries for can be worth it.