What if someone decides that he wants to inherit your class, even though you didn't plan that? Personally I believe users of my module to be competent and not seldom more competent coders than I so there's a great risk I'll overlook something that at least some users will see. So I try to not paint myself into corners if I easily can avoid it. This applies to more than inheritance and accessors.

Why pay the price now if it's easy to change the code later?

For a user that wants to extend your module it's not easy. In an ideal world the user would send you a patch and then write his extention, but we're not quite there yet...

One should be nice to one's users! (At least if it hardly costs you anything.)