Actually, it is. Mouse buttons exert a stronger force when held down, but (almost all) keyboards actually take less pressure to keep held down when they are fully depressed. Mechanical keys, for instance, have a pressure threshold that, once reached, activates the key and the mechanism drops most of the pressure. Not to mention that you can press a key with any finger and still be comfortable, but if you try to use a different finger on your mouse, it is likely to be uncomfortable, especially when you have to move the mouse around. I guess if you have a special mouse it may work better, but all the mice I've used have fairly high pressure to click (compared to keys). I'm just arguing my point based on what I've observed.
I think everyone should have freedom to set their keybinds to whatever they like, but I am just making a side point about ergonomics.
At least my middle finger on left hand gets more worn when moving forwards from W than the middle finger on the right hand pressing mouse 2. But I've got years of experience of using my right middle finger. That's actually the finger I usually show to people when I need to correct their behavior.
