We have three contracts - two at BWV and one at VWL - all with September use year. It does make a difference for us in tracking. Makes it much easier as we have the the same member number for all three (even though two were resales). It also makes banking deadlines very simple to calculate.
They are all considered separate contracts for selling purposes, but in the computer both BWV contracts look like one lump sum, so we can use the points across nights and such just as if they were one big contract. But if we ever decided to sell one, we could do so.
For us it was just a convenience issue. The only time you have to match your original use year is if you add on less than 150 points from Disney directly. More than 150 points is considered a new contract and you can do what you want, but less is an add on and they will force you to match it.