DCVS is good because you can have tracked local changes. Sometimes you are doing a larger change to the software that takes a while and may require some thought

DCVS is good because you can have tracked local changes. Sometimes you are doing a larger change to the software that takes a while and may require some thought, without stacking up hundreds of minor changes sitting in the source folder uncomitted that get applied simultaneously (a la CVS and SVN) and merges happen into that working folder, with no reversability. You could make the same argument all the way down to requiring that every single line changed is committed at the instant of writing (otherwise they are hiding their code!). Without tools like Git and Mercurial, people are still doing exactly the same behaviour, just it isn't being source controlled.