XI vs Traditional approach in connecting 2 R/3 systems in same landscape

Hi,

What would be the best approach to connect two R/3 systems within the same landscape to exchange IDOCs/RFCS.

All these days, we never needed a middleware to connect two R/3 systems and I had witnessed no problem in doing so.

With SAP XI in place, is it expected to route all these idocs via XI. What is the advantage we get ? Other than documenting the integration scenario... I am not able to find any reason why this should be so. Wouldnt using XI be a more performance burden approach?

Or we shouldn't use XI in such cases?

Thanks!!!