In no way do the items that come off the cornucopias hurt the game. Let them spawn everyday. They try to do something like what you want and many more problems can pop up with items that spawn like the horns. If it makes rare cornstalks worth less, so what.
I didn't say that they hurt the game. I was just concerned about those items which seem to be fairly Thanksgiving Day/Autumn oriented, like harvest wine, etc. In fact, I wasn't so much concerned about the value of the contents of the cornucopia as much as I was the value of the cornucopia itself.
If you take away the functionality of the cornucopia except during the holiday season, many folks who might otherwise hoard them may no longer see it as a storage investment. In the end, when the next holiday season rolls around, the presence of the cornucopias would be seen as a visible cue, rather than just another housing deco. Think of it like turning on or off the Christmas decorations on all of those trees.
Anyways, it was just my opinion. Certainly not an attempt to "keep the playah's down".