Not only do homes provide safety and refuge from Dragon (and the owl on a bad day) and other dangers, but they also tell us a lot about the characters living in them. Mrs. Frisby's home is snug and comfortable, and we know she would do just about anything to protect it. The rats' home is palatial compared to the Frisby's cement block, and its size and grandeur tells us just as much about the rats. Even more importantly, home is where the people (animals) you care about can be found, and all of the characters in Mrs. Frisby and the Rats of NIMH would do anything to protect those that they love.
In Mrs. Frisby and the Rats of NIMH, homes tell us something very important about the characters who live in them—what they hold most dear.
Even though the cinder block is cozy and homey, and the rats' burrows are impressive, in the novel, home is much more about the relationships of the characters than about the physical spaces they live in.