There is always an exception to a rule. I have trained with a few good "tactical" knife guys, and each one had me doing some eye-opening drills in scenarios taken from real-life altercations; elevators, getting grabbed from underneath a car while getting in, getting hemmed in by two "burly thugs" and "steered" away from the public, etc...
Probably the most interesting for gunfolk was the original Spyderco clip-it course offered by Erik Remmen. That guy will have you believing in carrying a small mousefolder on each side pretty quick with some demonstrations of the above scenarios. And, yes, weapon retention was a big part of the weekend.
It's another tool. It goes, as the mods say mindset, training and tools. A knife can be a great tool, if you know what it's good for. In trained hands in an elevator, it's hard to beat SHTF style.
Plus, knives are cheap, and the classes are cheap too, by comparison to a good shooting class. No reason not to be well-rounded, IMHO.
Other opinions might vary.