We started developing a system along these lines of thought about a year ago, and despite my dislike of XML I initially thought that it might actually work?!?!?!
Sure enough it worked beautifully in a limited testing environment. Then reality of production loads hit us straight between the eyes!
Due to the bloated nature of XML the cost of sending and parsing the XML messages FAR outweighed the cost of a simple ADO call to the database, particularly if that call is to a well thought out stored procedure. It may be that in cases where you would have to do multiple INSERTS/UPDATES etc... the XML option might be a break-even proposition, but overall it was a dog and killed our limited network bandwidth to our remote branches.
Reducing round trips is all well and good, but if you replace 10 round trips with a single, inefficient round trip you are not buying yourself anything. You might be giving up the very efficiency you are after in exchange for nothing more than the latest industry fad.
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If most people are not willing to see the difficulty, this is mainly because, consciously or unconsciously, they assume that it will be they who will settle these questions for the others, and because they are convinced of their own capacity to do this. -Friedrich August von Hayek
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