Other than when my points become available, does the use year have any impact on making reservations or using my points? We currently have a Dec use year.
UY has nothing to do with when you can call or go online to book. Your pts are available to use when you are 11 months out for your home resort or 7 months out for other DVC resorts.
(You start by looking at the dates of your stay and then looking at which UY (not calendar year) that those dates fall into - if you have a Dec UY, then a July 14, 2017 stay falls into your 2016 UY (which runs from Dec 1, 2016 to Nov 30, 2017) - which tells you that you can use current 2016 pts when you book the stay in Aug 2016 + banked pts from the previous (2015) UY, if any + borrowed pts from your 2017 UY if needed. It makes no difference that when you call or go online in Aug 2016 to book the stay, that you are still in your 2015 UY.)
UY is mostly an issue if you need to cancel a reservation. Ideally, you never want to travel in the last 3 months of your UY...so with a Dec UY, you would be putting Sept/Oct/Nov stays at extra risk if you needed to cancel them after your banking window has already closed (i.e. after July 31 of the next calendar year for a Dec UY).
So if you preferred to travel in Sept/Oct/November, then it might not be a bad idea to pick up a different UY to cover those trips.
But for most people, it's a lot more simple to stick with one UY for all contracts...