[plt-scheme] exception-handler prompt

I'm pretty sure it's never been possible to set the 
current-exception-handler mutably at top level and use it without at 
least one surrounding with-handlers. If I'm reading the error message 
correctly, in 359.x this manifests as a "missing prompt" error:

 > (define f (with-handlers ([symbol? (lambda (x) 42)])
               (current-exception-handler)))
 > (current-exception-handler f)
 > (raise 'boo!)
exception raised by exception handler: abort-current-continuation: 
continuation includes no prompt with the given tag: 
#<continuation-prompt-tag>; original raise called (with non-exception 
value): boo!

What is the rationale behind not having an implicit, top-level prompt 
for exception handlers?

Thanks,
Dave