Everything I was going to say about leaving a spinnaker out has been said. The sailcloth degrades from UV rays, obv the coatings help but not that much. The constant moisture will also degrade the sail.
I'd agree with Ding that leaving a spinnaker (or any sail) outside would decrease it's life by about 50%

My prefered method of storage is in the snuffer bag so it is not stuffed into a small bag. I do however either take the retrieval line out or make sure there is no tension against the sail.
Flaking is better than stuffing it into a small bag but i'd prefer rolling or keeping it fairly loosely stuffed in a bag.