I'd be interested to know which scenarios you are needing to access loose resources for.
Webuilt our DeploymentCatalog based on leveraging the core APIs exposed by the runtime which do not surface loose files. If they had been available we might have considered it. We are looking in the future at having better support if the runtime exposes
APIs for that.
In terms of whether or not you should use DeploymentCatalog or not, obviously that depends on your scenarios. It is a fully supported part of Silverlight 4 and to date we've heard very positive feedback from customers using it internally and externally.
We've also not heard of lack of support being a major blocking issue as embeddded resources are supported.
Thanks for the feedback