For decades, consumers who wanted just a few channels had to pay for all of them. Comcast or DirecTV couldn’t offer, say, MTV without also including Viacom’s less popular channels such as TV Land. While consumers wanted to pay for single networks on an a la carte basis, the industry fought attempts to break the bundle into smaller, less expensive pieces.
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.