For us, the best strategy seemed to be to tell the customer service folks we talked to that we simply did not watch the service at all, thus it didn’t matter how cheap they made it. This was a true statement, after all – we hadn’t watched it at all for weeks prior to the call. While they gave us some pushback and tried to make some lowball offers, I simply responded with, “That’s great, but we just don’t watch the service at all,” and it’s really hard for them to say anything about it.
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.