Using a sling inexpertly and en masse is a lot easier than using a bow inexpertly and en masse. You don’t have to be able to kill a rabbit at two hundred meters if all you want to do is cause some disorganization in a pike block.
But I really don’t think you could be doing even that with a big, heavy shield on your arm. Maybe slung over your shoulder?
We get slapped around, but we have a good time!