Hi Mohan,
Pros:
SAP PI is a middleware and all Middlewares work pretty much on the same principles.
I am assuming that EDI 850 would have some sort of structure, [ to be honest I have not worked on EDI but just read about it] and SAP PI would require you to convert this structure into XML -> the only language SAP PI understands.
Now, I am sure there are lots of third party adapters available in the market for EDI. PI will give you flexibility to convert this interface into any kind of requirement be it JDBC/FILE/IDOC or anything and vice versa is also true. PI give you this enormous ability to convert to and from any format as long as it is viable.
Last but not the least, it will provide a robust system with Alerting and Monitoring.
Cons: You may have to invest on a third party adapter to read the EDI
You can read the following links for more abt EDI and SAP PI:
Thanks & Regards,
Tejas Bisen