The situation here is not unlike the problem with null: If you allow null in your language, you will have NullPointerExceptions at runtime. Unused local variables (and other unused constructs) are known to be correlated with bugs (see e.g., Xie and Engler, 2002), so if you allow them, you will allow these bugs. So, by enforcing the absence of unused constructs, a large class of bugs is eliminated.

But I agree, this can be cumbersome. So, Flix allows any unused construct to be prefixed with an underscore to mark it as unused. (But underscored things cannot be referenced.) This seems like a pragmatic trade-off.

(I am one of the developers of Flix).