Ryumaru: I'm not disagreeing with you. When working on line-art or other two-colour art, not being able to paint different colours with both buttons becomes a hindrance. There's no reason you can't have it both ways though, for example: LMB - paint colour 1, RMB - paint colour 2, ctrl-LMB - pick colour 1, ctrl-RMB - pick colour 2
Ideally you could configure your workspace so you could have even more, attaching a separate tool and associated context (colour, brush, dither pattern, etc.) to any input event (keyboard, mouse, tablet, gamepad, etc. and combinations thereof), and so further reducing modality.