If I have a seed in addition to a 2-of-3 vault, it would be like a 2-of-4 in a sense, wouldn't it? That is: with a seed, it's as if you had the original wallet.
But it's also another complication since I need to find where to hide this seed. And I think it's already quite difficult to manage the 3 different locations with the 2-of-3 multisig.
The desktop of the 2-of-3 is not airgapped. Each element of the 2-of-3 vault has its own weak point (the hardware wallet is the stronger element, though), sure:
The desktop can get a virus and be stolen, as well as the mobile phone. But what probability is that both of them got stolen or got virus/keylogger in the same time?