[iradik]

Gonna play devil's advocate here:

I think making files in a folder represents the least amount of effort for making a queue. So using one of the systems you described is necessarily more work.

I think the commenter outlined the reasons: any process can access the data with simple unix commands, and everyone understands files.

Plus files could be more efficient. What if the work unit you are processing are files? If the files are the work and the folder is the queue, you don't need any extra abstractions to access the data.