My plan is to use sling and alternate between Amazon Prime and Netflix for certain things. I don’t need them all year. Amazon Prime will be a month around xmas, and then 6 months later. Then 1 month for Netflix to catch things I want to watch there in between. No need for a full year. I may use sling. Depends on what all channels and shows I can find there. I have never had much luck cutting a deal with comcast though, and I will still be dependent on them for internet.
The question of "when does it make sense?" can easily be rephrased as "how much money can I save?" and one of the biggest factors will be the price difference between combined internet and TV service or just internet. These are also the options that vary the most widely based on where you live, and the availability of promotions or contracts that can keep prices down for a short time.