There are MANY old, outdated "truths" that are perpetuated by instructors. The LE, CC, "defensive/tactical" and even the general military training communities are VERY slow to change, adapt, and modernize their training.
If these communities were as willing and able to adopt new, modern, and more effective techniques as the "competitive shooting" and "military spec-ops" communities, MANY of these old "truths" would enter the dustbin of history more quickly.
People like Todd Jarrett (a VERY good IPSC shooter who is now semi-retired and does training for civilians AND military) are trying to bring these more modern techniques--speed reloads with magazines, tactical reloading, shooting on the move, and techniques for transitioning to another gun--to the LE, defense, and military worlds, with a margin of success.
Look into trainers like Jarrett. There are a LOT of techniques for self defense that can be borrowed or adapted from modern competition shooting--target acquisition, field of fire issues, cadence, movement, and ESPECIALLY reloading are all things that the competitive is DECADES ahead of most "real world" training programs on--including LE, CC and the general military training.
But its an uphill battle. Some of these "truths" are so old, and so ingrained in the "self defense instructor world" that it will literally take an entire generation for them to be purged from the training practices. The idea that it's easier to grab a BUG than reload a modern semiauto is simply foolish on MANY levels, and needs to be abandoned. Putting away a perfectly good 1911 an grabbing a BUG just to avoid doing a reload is just ASKING for trouble in a serious SD situation. The possibility of fumbling one or both of the guns, the time re quired to establish a new grip on a different gun, and issues with recoil and recovery time differences are just a recipe for disaster...
Carry a BUG if you want. But one or two spare mags for your main firearm are ALWAYS going to be a faster method for getting more lead downrange.
The only time when a gun-switch would be preferable to reloading your main gun would be if one hand was incapacitated in the fight...