I am convinced there is a subsection of Murphy's Law that states "The chances of your next urgent repair scenario requiring the parts you have so carefully stockpiled is... not quite zero, but getting there."
I have storage boxes full of parts, used and new ones I got on deals I couldn't pass over.
In the last two years, I have had to use: a set of glowplugs
.
Honestly, as others have pointed out, an entire spare manual trans would be low priority, unless yours is giving hints of the beginning of a problem.
Now a known-good spare automatic... that would be a different story.