Typical speeds (speeds that most users can expect during peak usage) are almost always lower than the maximum advertised speed across all providers. Cable providers in particular tend to have lower actual speeds than advertised. Speeds will vary by location. When you enter your zip code above you'll see actual vs. advertised speed for each provider in your area.
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.