I admit I'm a bit sensitive to this subject. I was brought up in a congregation that disbelieved anything traditional medicine had to offer. They believed in every single type of nontraditional thing out there, with outlandish claims leading people to make poor decisions.
A few yrs ago I took a vitamin suggested by a nurse practitioner, at the dose she recommended. I confirmed the dosage and use in holistic books too. I have a health condition that unbeknownst to me was significantly worsened by this vitamin. I truly almost died. Very scary.
When I was working in psych we saw many people hospitalized because they thought they had depression and treated it herbally, triggering manic episodes.....they hadn't bothered to see a physician to get a proper diagnosis and didn't realize they were bipolar. These were people that would not have ever needed hospitalization, just proper outpatient care.
My $.02: There is nothing wrong with trying adjunctive treatments as long as they are totally safe for you personally, they don't interfere with your health in any way, and proven effective treatments are not being ignored in favor of something "natural".
Problem is, people love to say, "It's good for you." Truth: There is not one substance on the face of the earth that is good for everybody.
I am much more likely to try an adjunctive therapy if it has been proven by the scientific method instead of anectdotes passed on through generations. Then we could tell if it was the cinnamon speeding the healing process, if it was some other random factor, or if the body just naturally fought off the cold.
If cinnamon is safe for you personally, you're probably not doing yourself any harm by trying it. It may or may not be helping you though.