Your notion also intimates, a bit, that somehow owning a horn which is 'different' in tone, ergos, blowing response from your main horn is gonna hobble the player should they have their main horn unavailable. That's pretty arguable.
If someone has a second horn, they like it, they woodshed with it a little....then once in a while they probably will take it to a session or gig and play it. Once that is established, even if it sits in the closet for 7 months before they use it again, the majority of players will not be 'set back' by picking it up again.
The quality of "one that you can blindly play as well as your regular horn" is 100% up to the player, not the horn model.
Many a player with multiple horns chooses the second horn for a particular context BECAUSE they are gonna play it in a certain way which adds a certain something appropriate to that particular session/context.
So if you have a Yani you love - and you have a chance to acquire a 6M, you aren't gonna nix it because 'it's so different, I am gonna assume that (after owning and playing it for a bit) I could not pick up the 6M and be really happy playing it at a session or out".
It gets into the assumption of the backup horn needing to be in some way 'close' to the main horn. Again, a debatable notion to which I would say is easily trumped by the opportunity to expand ones experience with other horns
Mblock has arguably the classic French sax. Now he has an absurd opportunity to buy a classic American one. If it doesn't work out, he probably walks away making a profit.
My 2 cents.