According to my knowledge in SAP XI we have so many kind of 
control levels to process the message. For instance i will 
discuss on few points, like
1) Here using XI we can communicate with the SAP or Non-SAP 
at sender side or reciever side.
2) XI supports both Syncronus and asyncronus communications
3) in XI we can find the status of the message where it is  
   during the process of Message processing.
4) We have so many kinds of monitoring techniques.
5) Here we can have N number of senders and N number of 
   recievers based on your requirement.
6) It supports to process multiple messages at a time from 
   different senders to multiple recievers.
So I feel we cant perform all these things in 
Interfaces.That is the reason XI come into picture.

Still if u get any thing other than this please mail me at 
cravisap@gmail.com

We can say Interfaces is nothing but conversion programs 
in BDC.(ABAP).Here the data is stored in the file format 
only(Binary or text)...

  We can provide interface between two systems(sap to non-
sap)or(sap to sap)using the ALE process. Here the data 
stored in the idoc format only. This is also related to the 
ABAP.

   We can provide the interface between two systems using 
the EDI technology also.But it is not related to the 
ABAP.Some of the EDI developers are working under this 
environment in the Organizations.This technology understand 
file format only.

 XI/PI ROLL IN THE INTEGRATION PROCESS:
----------------------------------------

   While coming to the XI/PI.It is one of the middleware 
technology to exchange the data between two systems(sap to 
non-sap)or(non-sap to sap) or (sap to sap) or (non-sap to 
non-sap).Most probably non-sap to non-sap interfaces work 
with TIBCO technology in the Organizations.The 
functionality is same while using the XI/PI,ALE and EDI.By 
using these we can provide interface between the two 
systems.

   But using XI technology it understand any format.Assume 
we are created one idoc in sapR/3 system.This idoc want to 
use Oracle people with their database.Here XI converts the  
idoc format to SOAP-XML message (Because XI understand XML 
language only)  and using mapping program in XI it convert 
into the Database message(in SOAP-XML)and using the adapter 
configuration it convert into the Database specific format. 
This is the process flow for every Interface in the XI.

   The main advantage with XI technology is XI server 
supports for millions of messages at integration. Cost wise 
also its cheeper and relaiable. So now a days number of 
clients are willing to implement the XI for integration of 
the systems.