I can't imagine feeling the need to stock a spare capacitor or contactor when I can get one on one or two days in the mail.
Well, if you live in TX or FL or anywhere on the sourthern coast states, it may be wise to have spare.
Especially if the AC gave up on a >100F days (plural).
I personally, test the capacitor every spring season when the AC starts to be used to prevent failure possibility but I still have spare.
However, the service contractor should always have stock in their truck so that may not be necessary.
As for what they have in their truck is a mood point when we need it badly.