This definitely depends on your situation.
If your furnace is relatively young it's probably not going to require major service anytime soon so you may not need it.
As @sdg said, if you can tolerate a couple of days with no heat, you may not need it.
If you have access to trusted HVAC repairman, you may not need it.
Mind if I ask how much they're charging? There's definitely a pain threshold that will vary based on your needs.
For example, I'd be willing to pay up to $10. But that's mostly because of the annual maintenance check. If I had an older system or depended on it more, my pain threshold would be higher.
My local gas company offers this service for around $20/month. That's not worth it to me because I'm estimating (i.e. guessing) that any repairs I need to do in the next 5 years will cost less than 5*12*20 = $1200.