Remember that the person changing your oil at the dealer will not be their certified mechanic, but (wait for it......) an oil changer. Just somebody that, quite possibly, will be making close to minimum wage.
With that being said, I'll give you the one experience that I have with "free" oil changes.
In December of 1987, I bought a brand new 1988 Cougar. This dealer, in the Dallas, Texas area, threw in free oil changes (every 3K miles) for the life of every new car they sold. I tried to get them to lower the price of my car if I could opt out of the "free" service, but they explained to me that it was company policy and they could not exclude me from the services that they threw in. So 3K miles later, I brought it in for the first oil change. They finished it, I get back to my apartment, park the car, and notice a big puddle of oil under my car the next morning. A quick look underneath reveals that it is coming from the drain plug. I drive it down there to show what had happened and they bring it back in to their shop, drop the oil and luckily, it just took a new drain plug to repair it. However, as I watched through the window of the customer waiting area, I noticed they had dropped in oil into a dirty container and then poured the oil back into my car. They gave me my car back, without an apology of any kind, and let me by on my merry way.
I never took them up on their free oil changes again.
That day I had lost all confidence in the oil changer, the shop foreman and the cheap dealership owner.