"Is Quad Curve an ultimate EQ ?"
To some users, I would guess, yes it is. But for others, we have and want and like several different EQs. Do we "need" other EQs? Well, there's the question.
I use the QC on every mix in one form or another. Mostly for surgical (fine tuning) adjustments. I really appreciate the spectrum analyzer being included. But I use many other EQs as well through out a mix.
Afrodrum
QC's 4 bands, Q range + HPass & LPass, beatiful GUI, seems perfectly enough for any task. Are you aware of any EQ plugin which job could not be done with QC ( or QC/Sonitus/team). I guess EQ is only supposed to cut or boost, if its coloring the sound then that's filtering or distortion, not eq'ing.
To my ears, different EQs sound different. Is that filtering or distortion? Is it just different EQ curves that could be emulated in the QC? Perhaps, but I doubt it. I like hardware emulations of different EQs. I like using the EQ section from a Neve 1073 or API 550 on vocals, for instance. Can I recreate that sound with the QC? Maybe, but I'd rather use the "real thing" than spend my time trying to make QC sound like a 1073.
Have I spent money on plugins I don't need or don't use? Absolutely!! But not on EQs. Both software and hardware. Same with compressors. Different tools for different jobs and all that .....