The only real drawback that I see is that if you borrow for each trip and ever have to cancel, those points can't go back and must be used by the end of your UY.
If you have the flexibility to travel at different times, then it might not be that big of a deal. But, if you are traveling on borrowed points toward the end of your UY and something happens, even if you change your resssie or WL that uses less points, there may not be enough time to use up the points.
I originally had an Oct UY and we travel in August. I realized that if I ever borrowed for that trip and had to cancel or change (at least 31 days out), I would lose the points since I am a teacher and can't travel in September.
So I know have a June UY so I am traveling at the beginning. But as far as borrowing the 100, it depends on your contract and how many points you are using each trip. If you are borrowing because you don't have enough yearly points, you will run out a lot sooner then the end of your UY.
For example, if you have 160 point contract but borrow 100 for a 260 point trip in 2009, you will only have 60 when 2010 starts. If you borrow 100 again, that will give you your 160 back--not enough to do the same trip.
But, if you are borrowing but staying below your yearly allotment of points, then you are right, it will affect you a lot farther down the road.
Good luck!
