At the same time, you have to take the opponent into consideration. I'll use your mk/fox both even vs falco example.
How many mks do you think your opponent has played? Probably a ton since hes so common.
Now, how many Foxes do you think your opponent has played? In most cases I'm willing to bet no more than 3 because fox isnt popular in tourney. This is assuming he doesnt have a fox for a practice partner. I've seen this scenario unfold oh so often, even in my sets. For example, I'll start as falco vs d3 as either lose/barely win. Then I'll go to fox and completely blow the guy out of the water. Both characters do well vs d3, but still there was a significant difference. I know for a fact my falco and fox are about even.
In addition, against everyone I practice against and get to see all 3 of my characters, they usually do the worst against my falco/mk.
so my theory is that unless they know the lower tier mu as well as the higher tier one, having multiple characters can really give you an advantage and catch someone off-guard not knowing the MU.