1) It's important if you want to add on. If you have a a contract from a use year with a small number of points, then it's going to be harder to find a contract through resale that matches that use year. Because there are fewer points in that use year, the odds are that there will be fewer resale contracts available with that use year when you want to add on. For example, I own an October UY contract at BLT, which has the fewest number of points out of all of the use years at that resort. If you check resale listings, you'll see that there are very few October BLT contracts available at any random time, especially small contracts for an add on.
2) It's a little more complicated to book reservations that go over use years. You're right that you can borrow from the next use year in order to pick up the beginning of the trip, but you're then in trouble if you have to cancel the trip since borrowing/banking are permanent transactions that you can't undo. If you need to cancel, you may have points stuck in a use year with no way to use them.