I think it all depends on how long you plan on keeping this 12 year old car.
Since the water pump is driven by the timing belt, if you don't replace the pump now, and if it starts leaking later, you will wind up paying essentially the same labor costs later that you are paying now for timing belt replacement. Or, in other words, if you get the water pump replaced proactively at the same time as the timing belt, you will pay little labor cost for the water pump's replacement. If you are planning on keeping this car for more than another year, I would say that this would be economically prudent.
However, you may like gambling on the chance that the water pump will not develop problems while you own the car. That could turn out to be a good bet, or it could turn out to be a costly one. After all, you are now about 3 1/2 years late on replacing the timing belt, so maybe you will luck out with the water pump the same way that you have lucked out with the timing belt.
I think it all comes down to how risk-averse you are.
Personally, I would replace the water pump, serpentine belt, timing belt, and all belt tensioners at the same time. Paying a bit more at this point could wind up saving you money over the long term.