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