Hi Andy.
There are several reasons: FTP does not grant the delivery whereas messaging systems (TIBCO, IBM MQSeries, etc) usually grant you the delivery of the message. In addition, messaging systems are asynchronous and provide you some additional features like return receipt, request/reply functionality, pub/sub, etc.
You can browse the link below for an interesting thread on the topic:
http://www.mqseries.net/phpBB2/viewt...d52143e6880223
Cheers,
Testo