Not sure what the service is, how much it would cost you or what is included.
My TWC includes on-demand for all digital tiers. Since the old analog tier is rapidly disappearing at the rate of a couple channels/month, I'm not sure I see a difference.
At least in my area, most of the on-demand content is time shifting of shows that are on regular cable channels. I don't use it much because the on-demand content is not as flexible as recording and then playing on a DVR.
Now there is some content that is only available on-demand. You can only get HBO-OD or Showtime OD if you subscribe to the regular channels. There is also some stuff like Howerd Stern. For that stuff you need to decide if the content itself is worth it. I never listen to Howard, but if you are a fan, it may be worth it to you.