I think most people don't understand just how underpowered a multiclassed caster can be.
By losing caster levels, I have effectively neutered myself against things that have SR. But because I have lower spell levels, my saves are also lower, meaning I can't hit things with decent saves either. And with globes of invulnerability and the like, many of my spells just don't work. So a multiclass spellcaster can't use offensive spells.
Okay, I'll use defensive ones. But unfortunately, at higher levels many of those become obsolete. Invisibility is easily negated at high levels, buffs spells (because of your low caster level) are easily stripped away by dispels, spells that give +1 or +2 benefits just don't cut it anymore in a world where +4 and +5 are now the norm. In short, low level spells don't cut it in the high level world.
Now going back to the arcane trickster example, where the person kept saying their player had no problems with the weaknesses of his spells. For a person whose already willing to take the penalties of multiclassing and such, then the argument is pointless. If your already willing to multiclass as a spellcaster go ahead. The problem is that the majority of people once they have a little experience under their belt, recognize that multiclasses casters are much weaker than most archetypes and so will never play them. This feat at least gives them some incentive.
The feat is not overpowered, it just that the archetype is so weak in general.
By losing caster levels, I have effectively neutered myself against things that have SR. But because I have lower spell levels, my saves are also lower, meaning I can't hit things with decent saves either. And with globes of invulnerability and the like, many of my spells just don't work. So a multiclass spellcaster can't use offensive spells.
Okay, I'll use defensive ones. But unfortunately, at higher levels many of those become obsolete. Invisibility is easily negated at high levels, buffs spells (because of your low caster level) are easily stripped away by dispels, spells that give +1 or +2 benefits just don't cut it anymore in a world where +4 and +5 are now the norm. In short, low level spells don't cut it in the high level world.
Now going back to the arcane trickster example, where the person kept saying their player had no problems with the weaknesses of his spells. For a person whose already willing to take the penalties of multiclassing and such, then the argument is pointless. If your already willing to multiclass as a spellcaster go ahead. The problem is that the majority of people once they have a little experience under their belt, recognize that multiclasses casters are much weaker than most archetypes and so will never play them. This feat at least gives them some incentive.
The feat is not overpowered, it just that the archetype is so weak in general.