Strange Planet by Nathan W. Pyle

Everyone focuses on why learning metric would be better in the first place, and they're right. No one has come up with a good argument for me to throw away all of my measuring tools, convert all my recipes and relearn an entirely new system when the system and stuff I have works for me now.

Those aliens have 3 fingers. A decimal system to them is like a system based on 14, 196, 2744, 38416, ... would be like to us - probably worse than US Customary