The first and primary reason for having a cover on all of your junction boxes is safety. First of all, wire splices are kept in junction boxes - live wires that can come into contact with humans and animals alike and cause serious injury. The junction box covers are there to protect people from being able to touch the wires. In addition to that, they are there to protect the wires from damage too.
In addition to this, wherever wires are spliced together heat is created from the restricted electrical flow. If the wires ever got hot enough, the first line of defense against the rapid spread of fire is the junction box cover.
So before you ignore this "minor" maintenance item, consider the very small cost of correction compared to the possible serious issues that could occur.