If someone prepares a pull request with a well-structured series of commits, making a logical series of changes, where the project builds and passes tests after each commit, then those commits shouldn't get squashed.

However, I frequently see people adding more commits on top of a pull request to fix typos, or do incremental development, where only the final result builds and passes, but not the intermediate stages, and where the changes are scattered among the commits with no logical grouping. In that case, I'd rather see them squashed and merged than merged in their existing form, and having a button to do that makes it more likely to happen.