You're right, you can't represent .9999... exactly in a computer. Neither can you do so on a piece of paper. No matter how small a font you use, you will eventually run out of paper. The best you can do is talk about it, repesent it in various ways, but what you can't do is actually write it out explicitly, make it "real" if you will. Numbers manipulated by calculating devices are restricted to a finite size due to physical limits. Consequently there is a finite number of numbers that can be stored. Therefore a computer is an invalid context for this discussion. .9 repeating to a machine's physical limits is not equivilent to .9999... .9999... simply can not exist in the digital realm. So you are not refuting the argument at all.

Have fun,
Carl Forde