What this suggests to me is that phantasmal force and similar spells occupy the same mechanical space as skills - in particular, social skills and the like - rather than attacks. But no version of D&D has really grappled with self-consciously balancing spells against skills, though 4e perhaps comes closest (but not terribly close) with its Ritual rules and its wizard cantrips.Say, you're looking at illusion spells -- something like minor image.
To keep it true to the play experience that people expect out of D&D, you need to have a flexible, general kind of spell rule for that. It needs to be able to create an illusion of whatever the user desires.
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Of course, this makes it nearly impossible to balance. Sometimes that little low-level spell will negate an encounter. Sometimes it will have nearly no effect.
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It's not exactly like you can balance damage-per-round against that. It's effectively infinite damage sometimes, no damage other times, up to DM interpretation almost always.
(And I would note that a PHB wizard has audible glamour (sorry, ghost sound) as an at-will ability - which is one important component of the low-level illusionist's repertoire.)
And in light of some recent posts, I want to stress this is not a "bash pre-4e editions" post. I've GMed AD&D illusionists, and also Rolemaster illusionists (similar in some ways, different in others) and they're fun. But if we're talking about mechanically balancing them, it's important that we make sure we're looking at the correct part of the overall design space.