Why should you use MTOM and not WS - Attachments ?  The answer is limited composibility and better performance with large attachments.

Composibility is the ability of a specification to be effectively used by specifications higher up in the web services (WS - *) stack

The composability characteristic of WS – Attachments / DIME is considered poor because of the non SOAP envelop approach to messaging. This makes it hard for Ws - * specifications higher in the stack to work with it. MTOM has better composibility characteristics. 

The moral of the story is to plan to use MTOM. On the Java side for interop, certain implementations support MTOM including the early release of WS02. WSE 3.0 and WCF support MTOM out of the box

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