If use is the only thing you do it may contribute to the Tragedy of the Commons and you undermine FOSS. If you also contribute back in some ways, even for very practical purposes, you are also serving the higher philosophical principle of FOSS.
The tragedy of the commons requires a resource that is being depleted, which definitely isn't something that merely using FOSS would do.
The effort of contributors can certainly be depleted when they see that no one cares to even submit a bug report or suggest any improvements
If you're using FOSS because it's convenient and don't have any problems, why would you suggest any improvements or report bugs?