Hi Juan, you need something in the middle to convert/transform/map the IDOC output to the cXML QuoteRequest. Likewise with the returning cXML QuoteMessage, it needs to be converted for the RFC/BAPI in SAP to create either the PO or OA. You could do this with other middleware products, but there will be a bit of custom dev to make it work. The advantage of using PI is that it is 'somewhat' plug and play, the mapping & transformation is taken care of by the Ariba Network Adapter.
Hope this helps.
Cheers,
Ben
www.dyflex.com.au