A good discussion..If I can add few things
1) PI REST Adapter is planned, should be available sometime in near future..
I do not think every integration scenario should be handled in Gateway (rather oData style) just because it can or it might work thinking. Doing it odata style means you need to re-write all functionalities as Web Based Services (eSOA anyone??)
2) Gateway (oData Services) - should be used in User to System scenario ( mobile, browser light weight type) , where you would want to transfer minimal data to the user and back
3) PI - System to System as you mentioned. e.g. Master Data/Customer Synchronization between ecc & crm etc. Doing them in oData style seems like overkill to me when you can use the IDOCs/ALEs and you do not need the data to be transferred over HTTP.
Both Gateway and PI are targeted for different scenarios and have their own place, and as such the right product should be evaluated carefully for each scenario.
Just my two cents.
Sandip