I don't think it's useless, exactly, but I do notice that checking the Lexile of a book tells us nothing about the complexity or subtleties of its theme or its overall literary quality. If your child has a high Lexile (DS8 got about the same Lexile as your son this year--something in the low 1000s, anyway) I think it means that they have a very good vocabulary and can process complex sentences well. I'm not sure what it means about what they understand at a higher literary level. But I have not seen the tests they use.
The test was slightly useful for us in that it did cause me to realize that DS could probably read just about anything he felt like reading that would be of interest to him. I gave him Tolkien after I got the results, and it was a big success.
