Ebay have just made these mandatory. There is no question that in the future they will be joining up listings with the same barcodes in some way. Either as shared listings like Amazon or as suggestions on the listing pages.
I have no interest in helping to promote other sellers selling the same items as me, therefore I would like to list with a different barcode to the ones on the products. BUT I also need to keep the proper barcodes in Linnworks for scanning and managing the items.
This could be solved by us being able to set ebay barcodes as an extended property (as we can do for Amazon through LL)