19:14:21sjl_It could break code that read-line's a stream opened with :element-type 'base-char but then tries to store a (and character (not base-char)) into the resulting string later.
19:14:37sjl_I have no idea if that's a common thing.
19:20:47sjl_So people writing (declare (type (simple-array character (*)) foo) aren't basing that on the standard, but rather just what SBCL has historically done