[Serialization] Why an xhtml output method?

I would be interested in an explanation of why an xhtml output method is 
included in the Serialization document and why it has been designed as it has.

On the face of it, the xhtml output method seems primarily to be designed to 
accomodate legacy browsers. I wonder if that is sufficient justification for 
its inclusion. If there are other significant factors then it would be useful 
if those were elaborated.

Thanks.

Andrew Watt
"XHTML 2.0 - the W3C leading the Web to its full potential ... to implement 
yesterday's technology tomorrow"