There are a few ways that it works in my mind. You have to be a bit nostalgic about your tools (and other inanimate objects in your life) for it to make any sense, so there's that underlying theme.
One view is that it's a permanent pivot in your tool rotation
based on some aspect of nostalgia rather than pure need.
As mentioned, another thought is to preserve something as an heirloom - either received or to be handed down.
A third is a deliberate change in tools to get maximum use out of a tool I like but have to special reason to carry.
I am in a similar situation as smiller in that my SAKs are similar - and my preference is for 2 or 3 layer models... especially 2 layer. When choosing between the Compact and the PioneerX, I had always picked the Compact. When I was in the mood for an Alox, or stronger tools, I picked the Soldier. The option of stronger tools and the scissors was there, but three layers always seemed to lose out to two. Putting the Solider on the shelf makes the 93mm Alox tools on the PioneerX a "unique" benefit. I like the PioneerX and would liket o get more use out of it.
My Soldier is in remarkable shape, and at some point, I plan to pass it along to an interested collector. I personally don't consider myself a "collector" as years, versions and variations don't interest me as much. Hopefully one of you may be excited to own a 2005 Soldier. So I want to keep it in as good a shape as I can.
Then there's nostalgia - some of my tools... my Champion, Wave, MP600, and my Soldier have wonderful stories to tell. So do my hiking poles, one particular pair of boots, coffee mug, backpack, guitars. old passport,... Among those, the MP600 sadly has the fewest stories but paradoxically had been riding quietly in the back seat for some of the best trips. Those days are gone. The stories for the PioneerX are just beginning.