Well, constant does say,

Even though a reference may be declared as a constant, the reference may point to data which may be changed

Again, for my personal need, 99% of my constants are simple scalars. Update: But then, most of my codes don't use constants at all. Just to be clear, I'm not by any means in position againsts Readonly. My first reaction to it was, "OK, cool, I'll might use it someday".