We're guessing that, since you're here, you probably care about reading a lot. But do you care about it as much as the characters in Mrs. Frisby and the Rats of NIMH do? For Nicodemus and Co., reading is about much more than simply learning a story. For them, reading provides them with hope and, eventually, the means to escape from NIMH and all its horrors. Still, even though reading gives them so much, it also proves to be a burden to the rats, as they realize that their new knowledge and skills mean that they will never fit in with others of their kind again. Knowledge may be power, but it can be a bummer, too.
Learning how to read was the worst thing to ever happen to the rats. Without reading, they would have been content to sit in their cages at NIMH, happy and well fed.
The best thing that could ever have happened to the rats was learning how to read. Once they learned how to read, they had the tools to change their lives for the better.