It's a lot easier to do things when you don't have to go "okay, what's the conversion between these two units again?". The problem isn't base 10 vs base 12, it's the fact that in SI, every unit scales in the same way (the prefixes). With imperial units, every unit scales differently. How many fl.oz to a tablespoon? No idea. How many teaspoons to a tablespoon? No idea.
Meh. Every 10F difference is a bit under 5C difference (since the ratio is 9:5). So you can estimate that 40F~5C, 50F=10C, 60F~15C, etc. All I remember is 2:1 and 32F=0C.