Additional note, April, 2015:

EWG has decided not to make a change in this area. See EWG issue 99.

Rationale (August, 2011):

The specification is as intended; changes to the restriction would need to be considered in a larger context by EWG.

It is currently undefined behavior to delete a derived-class object via a pointer to a base class unless the base class has a virtual destructor. It has been suggesed that this could be allowed for a standard-layout class. If so, presumably the caveats about a deallocation function or non-trivial destructor found in 7.6.2.9 [expr.delete] paragraph 5 that currently apply to incomplete types would need to be extended to apply to the derived class in such cases.

Another objection that was raised is that such a change would make it more difficult to extend C++ in the future to have global deallocation functions that can take the size of the object being deleted as an argument, as is currently possible for member deallocation functions.