Cutting down trees or mining.

[RIC0H]

It makes playthrough a grind.

How so?
Doing it would be completely optional.

If you don't want to cut trees, then don't. But it may be good for those that do want to do it.

You can always just stand in a room for hours on end doing nothing but contra, oh what fun that sounds like.
Me personally would rather be doing something to keep myself active while playing and also getting money from doing it.