Allocatable scalar components not allowed in derived types... why?

Just out of curiosity, could anyone enlighten me why allocatable scalars
aren't 
allowed in derived types (in F2003 or F95 + TR15581)? It strikes me as
this 
could be a useful way of having a component that either has a well-defined
value 
(ALLOCATED(foo%bar) == .TRUE.) or has no value (ALLOCATED(foo%bar) ==
.FALSE.).

cheers,

Rich