Sling is a good deal for serious TV fans, but if you’re not going to watch at least eight different shows on those channels per year, it’s cheapest to just get your Mad Men/Walking Dead fix by buying individual seasons on iTunes or Amazon Instant Video. That strategy, ironically, is pretty much what Dish Network’s chairman recommended back in 2012—before his company owned its own streaming business.
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.