Checking balance, image, etc... is part of process of
actual mixing.
One can
mix with headphones while
check something with speaker-monitors

There are other aspects and specifics of individual approach in the whole production process (sound experimentation, recording, stereo-imaging, mixing - all can be as one process). Also the question are: mixing what? (what kind of musical material?) and what for? (to be performed/listened where? on what sound-reproduction system(s)? and what for? (during what kind of activity, for example)).
One can produce material specifically (or primarily) to be listened on headphones.
Or if you take, for example, production of club/dance traks for club-soundsystems, or dub tracks for dub-soundsystem etc... then the process of mixing will have its own specifics and specific demands so you will need specific set of "tools" and use specific techniques, where your nice "industry-standard" studio-monitors will be nothing more than just another (maybe useful!) pair of speakers which may or may not be able to "give you information you need" in your studio. To get information you need - you actually have to play/listen/try your tracks in clubs (on club's sound-systems) and build-up your experience by comparing what you hear at your studio with what you hear in the club. In such situation: what ever works best for you in the studio IS your tool of choice - and , Yes, it can be a specific pair of headphones, studio-monitors or specifically designed set of what ever speakers, and YES, a pair of nice "industry-standard" studio-monitors in such situation CAN be totally unacceptable for the job and thus useless.
It's just simply not that
simple as ABC.
Well, it may seem to be simple if you view the mixing process from your rather "narrow window", then you MAY see
mixing as
hammering a nail but not as
building the wall. When building the wall - screws can (or even have to) be used. So what do you do with the "hammer" then?
/respects