I've never really understood this opinion. For me there's plenty of times that some problem needs to be solved and simply banging my head against it isn't going to make it happen any faster. Having some easy low priority stuff to do for a bit (or a high priority short task that interrupts) seems to let my subconscious solve the issue in the background and keeps me happier and more motivated. If it was two difficult tasks that were supposed to be done in parallel maybe that would be different, but I've never seen anyone asking for that