Why is 1:1 desirable for a macro lens? I know it means that you can print a picture of something and it will be the same size on paper as it is in real life, but why is this such a desirable feature over, say, a lens that magnifies even more?
You're close on what 1:1 means but slightly off. A 1:1 macro lens means that the size of the subject is projected onto the sensor (or film) at the exact same size it is in real life. You can blow up the print as large as you like :)
The ability to go to 1:1 is just a metric and there are a lot of compromises that come with being able to enlarge a subject to 1:1 the most obvious one being focusing speed since there is a larger range (distance) over which the lens can focus. There are lenses that go even closer, the MP-E 65 goes from 1:1 to 5:1 magnification, its a manual focus only lens though :)
The 1:1 designation means that the image of a subject projected onto the sensor (or film) is the same size on the sensor as real life, and is the minimum magnification to classify as true macro. There are lenses that do magnify more, such as Canon's MP-E 65 which can magnify images between 1 and 5 times their real-world size.
The benefit of 1:1 magnification is that you can truly enlarge and enhance real-world detail in print. This is in contrast with normal lenses, which often only magnify 0.15x, or 1:6. A print would need to be enlarged six times to present the same amount of detail as a 1:1 lens...assuming you could resolve that amount of detail in the first place.
There is nothing specifically desirable in a 1:1 magnification for macro. This is just a convention, where, as pointed out in Rowland's answer, to designate a "true" macro lens.
This does not mean that a lens with 1:0.9 max magnification cannot do a good macro work (if you can find such lens). I guess it depends on what exactly is it that you want to shoot. Depending on your subject's size, relative to the sensor's size, you can choose lenses with different max magnification.
Generally speaking, for insects, a 1:1 is a good bet. For insect eyes or sugar grains (...) an MP-E 65 may be better. For flowers, OTOH, a non-"real" macro lens can do decent work as well.