This queries outlined in this thread are to do with business-to-business transactions. Whatever about the wisdom of evading tax on business-to-consumer transactions, (which is not in any way recommended, due to the severity of Revenue interest and penalties whenever such a scam is uncovered), doing so in respect of business-to-business transactions is plain daft.
For a start, no business worth its salt will pay a supplier without being furnished in the first instance by a proper invoice, which would be expected to denote the supplier's PPS or tax reg number. This provides an obvious paper trail from the records of the customer to the supplier which can readily be followed by Revenue or any other authority. Secondly, if the supplier is unwilling or unable to furnish a proper invoice in respect of work done, they may have no legal right to get paid, as any contract between supplier and customer would be tainted by illegality.