Have I ever made a mix that sounded so good to me that I thought it didn't need to be mastered?
All the time
But in reality people who mix "usually" are not the best people to make those kind of decisions. It is entirely possible that a newbie mix could be to the point where one song doesn't need any addition signal processing, but the song would still need to be mastered. Sometimes in the realms of professional audio albums to show up that don't need any processing, but they still have to be mastered. Mastering is not EQ and compression exclusively.
Mastering is:
A second pair of experienced ears evaluating the music's ability to translate to the largest number of playback systems.
Knowledge of equipment and workflow to minimize the number of components in the processing chain to balance eq, tame transients and adjust levels, fades and repair glitches to name a few.
Knowledge of tricks to enhance weakness in the audio, like MS processing or removing excessive reverb, parallel compression, upward compression, and countless other intimate and discrete things that takes years of experience, thru failures and successs both.
A great pair of Mastering speakers in a room that lets you hear things you usually can't in a nearfield enviroment.
Mastering is also making sure your disc/tape that goes to the duplication plant doesnt have anomalies that cause system crashes and BLER count, whats tolerable and whats not. Does the audio null on loadback and things like that.
Lets not forget that you also need to know enough about equipment to debug glitches in front of an itchy client.
That should cover the basics on Mastering without getting into the cost problem of having both colored and transparent choices of EQ, Compressors, Limiters Etc....
SoMm