the only way this prepaid maintenance can possibly pencil out, is this:
dealer charges X for "covered" services, which they have, say... 30% margin on
dealer/contract provider knows you'll find the services for cheaper elsewhere, so offers prepaid service for X-15%
the 15% margin they lost actually secures them the additional 15% margin that they were never going to receive in the first place
in this model, you very well may save the % advertised (I made those numbers up), however the devil's in the details... read the fine print, make sure they don't nickel and dime the difference with non-covered consumables, disposal fees, etc etc.
one thing to always have in the back of your mind is that dealerships need lots of money to keep the lights on, and they're not in the habit of leaving money on the table. the more convoluted and confusing they can make their offerings, the easier it is to get extra money in the process.