Not OP, but I think that the idea is that if a notification makes it through the system, it has been filtered to be worthy of distraction. So, there isn't a cognitive cost to the user of "is the notification worth reading" since it has already been determined to be that way.

You're right. On average I probably get about a dozen spaced out during my working hours, 10am to 4pm. On a busy day I might get a couple dozen but the intervals make these easily manageable and I always have priority. If I see something from Gilfoyle in alerts that's probably more pressing than a company I'm following that just released unaudited financials in news. Setup might sound odd but in reality as said above anything that makes it through has some stake in something I'm invested in and want to keep an eye on.