do you think this makes sense? do you have a problem with downloading content which is solved by this exclusions?`
the main reason why these exclusions are there is to clean "time- and user dependent" properties from the downloaded content, so it can be safely stored in SCM without changing it's content just when uploading and downloading it.
dam:Thumbnails does not seem to be problematic to me, it does not contain any property.
mix:lockable contains two properties jcr:lockIsDeep and jcr:lockOwner which itself are not excluded - i've not seen this mixin often in sample content.
dam:Thumbnails is a property, that references the thumbnails of an asset folder. It indeed doesn't hurt, but I guess it depends on what you're using the plugin for: If you want to clean up the content, so you don't get changes all the time, then that exclude is probably not needed. If you want to clean up the content, also to not have auto-generated nodes/properties in SCM (that will be put back when they're needed, i.e. in the case of dam:Thumbnails and the dam:thumbnails node, auto-generated when the folder is viewed), then that's a different discussion.
With mix:lockable, I added it because it depends on authoring actions whether the mixin is added or not. But, it was a long time ago, so I can't remember the exact case.
@Stefan: I see in this file: https://github.com/wcm-io/wcm-io-tooling/blob/develop/maven/aem-global-parent/pom.xml#L433 also that the DAM renditions are excluded (but only the thumbnails, not the web renditions), so it seems we do at least something to also minimize the size of the resulting content, other than cleaning out the time- and user-dependent properties.