There's more abandonment in Maniac Magee than in an ASPCA commercial. Maniac's parents abandoned him by dying, his aunt and uncle abandoned him emotionally by hating each other, Maniac abandons the Beales out of fear, Grayson abandons Maniac by dying, Maniac struggles (and, uh, kind of fails) to not abandon Russell and Piper. See, Maniac isn't alone because he likes it. He's alone because people keep leaving him. Depressing? Yeah. But by the end, he finds a new home—and we get the sense that this time, it's a forever home.
The pain of repeated abandonment ultimately makes Maniac able to be a part of a functioning family. Without having gone through the struggle on his own, he wouldn't have the same appreciation for family life that he doe snow.
When Maniac sticks with Russell and Piper, he is starting to understand that families stick my one another without resorting to abandonment in hard times.