Why use a ten-percent rule-of-thumb for a bleeder current on a voltage divider?
This is actually an incorrect interpretation of the "10% rule", as I understand it. Or, perhaps, there are two of these rules used in different situations!

If you are simply scaling a voltage, you actually want your load current to be around 10% of your divider's current.

This is because you don't want the divider's output voltage to vary much when the load changes. And the load will change! Either because it is doing something intelligent (with a microcontroller, for example) or simply because ambient conditions (like temperature) will change.

Yes, this wastes a lot of power. This is the reason to use a regulator to power a load instead of a divider. If you like, please see this related answer to a different question.