No offense bro, but if you were good at math you'd know that though the calculations are trivial, in most cases you won't be able to do them in your head even IF you know the conversion values. By definition, anything that is easier which also provides zero loss of functionality is better.
Let me unpack this for you with two examples:
1) A feather is dropped on the moon from a height of 1.40 Kilometers. The acceleration of gravity on the moon is 1.67 M/s2. Determine the time for the feather to fall to the surface of the moon
2) A feather is dropped on the moon from a height of 203.2 fathoms. The acceleration of gravity on the moon is 1.2 yards/s2. Determine the time for the feather to fall to the surface of the moon
In example 1, you can convert 1.4KM to Meters simply by shifting the decimal point three places, them plug both values into your formula
In example 2, you will need to convert fathoms to meters (203.2 x 1.8288) and convert yards to meters (1.2 X .9144) then plug both values into your formula
You'll end up with the same answer in both cases, but why bother with the extra work and obscure conversion values when all conversion in metric is just a matter of shifting decimal places?