Hi Peter, there is very little downside to using a Business Object Library--really only that you have to deploy it, a very minor thing.We always use a separate library even for WinForms projects, because it does happen that one application is more or less a consumer of business object information, and then one day you need a maintenance application for this or that. Having the separate library lets you more easily reuse all that code.
In a similar fashion, every so often someone wants reporting or other information on the web taken from information used in the WinForms app. Once again, the separate library for business objects helps us react more quickly to those requests.