To say the truth, I think the spare battery dies only short time after the installed dies, if you buy it at the same time. The battery gets old and unusable even if not used just by aging effect.
The life expectancy of Lion battery is say 3-5 years. Can be more. I have iRiver mp3 player and battery still lives quite fine after cca 10 years.
M92 is not cell phone that needs to be charged every while. I would buy a spare battery 3-4 years after purchase of the device, if it is used daily.
Maybe users of M90 might share experience about battery life...
I am not sure if 2 year warranty covers also defective battery. I guess, battery should be OK during 2 years warranty (it is not replacable), if not it should be warranty case.
So, maybe wait with battery purchase at least 1-2 years.