In a post to the cocoon-dev mailing list with the challenging title of "Is Cocoon going to be harmful for XML?" Stefano Mazzocchi asks if XML server side transformations will not slow down the movement to more innovative client side XML architectures.

In a carefully balanced message, Stefano Mazzocchi's asks the Cocoon community if the project cannot be considered as a hack to take most of the benefit of XML on the server side, making less attractive a more generalized adoption of XML and potentially slowing down the movement toward client side XML:

While server side transformations allow new formats to be 'adapted' to older clients that do not support these formats, there is a great risk that, having this type of transformation capabilities on the server side, there will be less 'pulsion' to the creation of XML-capable clients.

This concern, which is common to other server side XML transformations, had been recently discussed on the XHTML-L mailing list.

The major arguments for server side transformations include the need for "semantic firewalls", and the requirements from web site designers to support a wide range of client releases, as mentioned in this answer from Donald Ball:

Stefano, in the real world, we're still supporting 3.0 clients and at least paying lip service to supporting 2.0 clients. Doing XSLT server-side is necessary, now and for years into the future.