Short URLs aren't just about twitter. URL shortening is useful in a variety of situations, and tinyurl was widely used for years before twitter came along.
In addition to space constrained applications such as SMS (and artificially space constrained applications like Twitter) shortlinks are also required anywhere humans are required to remember and/or enter them - that is, when they are printed, recited over radio or displayed on televisions. This is a very important use case that people often ignore.
