I currently have Netflix, Hulu, and Prime, with a number of "add on" subscriptions for Prime. This is one place I think Amazon really got it right. I login to Prime, and I can watch Prime specific content, as well as stuff from Showtime, HBO, Shudder and CuriosityStream, and it's all seamless. Plus if I want to pay for one of those "one off" movies that isn't part of a base streaming subscription, I can just click a button and it's charged to my Amazon account. I have my own issues with Amazon sometimes, but their idea of "add ons" really makes Prime a lot more desirable a service from a streaming standpoint.
Now the flip-side is, I don't really want to know how much all those little $4.99/month, $8.99/month, $7.99/month "add ons" are costing me in aggregate. :-)