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

Allowing for creative incubation is a really thing. But task restarting penalty is a real thing as well: you lose context and your mental model; working memory loses details; dependencies are reverified unnecessarily; flow state momentum lost; environment reconstruction each time and similar administrative tasks, etc.

> 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.

For a bit is ok. I had one month task, that was interrupted daily for over a year, that's how I got demotivated and burned out so much that I needed neurologist.

> 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.

Probably that's why you say interruptions are good, no experience of how bad can interruptions be.