Why is it a wrong practice to calculate voltage drop across current source?
There's nothing wrong with calculating the voltage across a current source.

But to do so you'd use KVL and find a sequence of other branches between its terminals, the sum of whose voltages would equal the voltage across the source.

There's no way to calculate the voltage of the current source just from knowing what the current is through it (or through other branches in the circuit like you would do for a CCCS). Therefore it's not useful in forming an equation for mesh analysis.