A few points not covered by the others:
The various focal lengths of macro lenses provide for 2 things - a larger image OR more working distance. With some subjects it is difficult (living things) or dangerous (an arc welder) to get close to them.
I use a Canon 100mm macro lens and at times add a 2x teleconverter to it (giving me 200mm and 2x the working distance).
I've also used the 100mm macro with a 3x teleconverter - some results can be seen here:
http://www.jbs-blog.com/?p=72
Macro lenses will focus out to infinity and can be used as general purpose lenses but the auto-focus in a macro lens is typically not too good (slow). Since most macro photography is done with a tripod and manual focus, I guess the lens makers just decided to not spend much time on it.