Especially when using mostly or partially intact chunks of compressed pu'er (depending on how tight the compression is), a couple rinses will be needed before the tea will release much flavor.
I agree that it's unlikely that a quick rinse with boiling water will remove all pesticide residue or rodent droppings, or kill any possible contaminants. However, one or two rinses may get some of the dust, bug droppings, or other small particles that may be in the tea, and may also help to reduce any storage taste the tea might have.
In any event, the tea police aren't going to come arrest you for drinking your rinse, so if you feel bad about dumping the rinse, and / or you like how it tastes... by all means, drink it.
I will admit that habit and custom are probably the main reason I rinse tea, though some of the other reasons people mentioned are also contributers.