Context A PresentationDesigner is creating or editing a template, not through the Zope Management Inferface, but through client software like GoLive or Dreamweaver. Such a file is no longer a valid TAL template. There is no consistent way to send back the intent of the rejection, so the user thinks that Zope is broken. Silently ignoring the error means bad pages. Bad pages mean bad sites.
Bad sites mean bad press.
The PresentationDesigner has a workflow. TAL is already disruptive enough to the workflow. In addition, using ZPT is also a change in the way things are done. The potential for more errors will cause the designer to question how useful doing templates and Zope is to begin with. In particular, TAL syntax errors will constantly remind the designer that they are working in a new paradigm.