In my environment I have a main upload folder and various project (-group) specific subdirectories as proper upload folders. This is required in order to allow for simple accounting of the data.

Therefore in my case absolute_path_default_upload_folder must not be writable, since attachments have to be reliably inserted into the proper (writable) subdirectories.
In that sense I only use absolute_path_default_upload_folder as a suitable start assignment for the project upload folder tree when I set up new projects,

When I execute admin/check the checker will prematurely stop due to failure with

absolute_path_default_upload_folder configuration option points to a writable directory

The path '<MAIN_UPLOAD_PATH>' must be writable.

Currently I chmod the main upload folder writable before running the check, but maybe there are better options.
I.e. the error level for absolute_path_default_upload_folder may be released to WARN, or the check routine could be implemented such that it does not stop at FAIL (which appears to me the better option in the course of writing this text)