I don't think I have ever met anyone who has a mini-14 and an AR in 556 and actually uses both of them.
Maybe if you have a target AR and a light sporter mini.
Like some others said I try to fill "needs," which are really wants. I try to keep cartridges, actions, accessories, etc as common as possible within that frame work. I end up with things like an Encore and many barrels for a shotgun.
But I also have more than one shotgun, even if they are all 12 GA