There are two reasons why model releases are a good idea:

1) they prevent claims for compensation in circumstances where compensation are appropriate, and
2) they make it much easier to sell or otherwise use your work commercially, even if the photo itself doesn't create a circumstance where compensation is appropriate (e.g. un-recognizable subjects).

The second situation is the more problematic - it doesn't do any good arguing with someone who won't buy your work because they (erroneously) think there needs to be a release.