I can see your point because you can also argue said point about FFVIII, levelling is nowhere near as important as setting your characters up with the correct junctions.
But still, sometimes I'd have good equipment and I'd still not find it worked enough, or maybe I just hadn't done enough, either way for me personally I feel that grinding a few levels doesn't hurt.
As for people who've beaten the game on a low level well good for them, I imagine that took a good few run throughs, knowing the game inside out and how to exploit certain things to make it work or several attempts in battles but that's not what I'm talking about.
I'm not sure if I mentioned this before but I've never beaten FFVI before so I'm talking about after a couple of playthroughs where I've attempted the game and this being my first one to fully complete the game where I'm taking my time with it and I still thought levelling was important, I've known that the Espers were important for their stat boosts rather than what they actually do, I don't think I've really used my Espers apart from stat boost this run through but I still wouldn't have known that equipment plays a major part in fact there's times I haven't noticed it.
The thing I did notice that doing a little bit of extra levelling along the Lethe River and a few later parts of the games before getting the esper stat boost did help me out a bit in parts where I'd struggled before that didn't necessarily have anything to do with what equipment I had. I'm just saying for a more inexperienced player it's not something I would take as much notice of as to someone who is highly familiar with the game.
Case in point again, FFVIII I had no idea my first couple of run throughs just how important Junctioning stats was, it was only after my maybe 4th/5th proper playthrough I completely mastered it and would argue for it the way you're arguing for FFVI.