The problem is that it is not enabled by the mediums we use to communicate.
and are copied and pasted from emojicopy.com and getemoji.com respectively. They appear differently on each website, and yet identical here when copied and pasted. It is not possible for me (or anyone at this time) to comment with two different graphical representations of the same emoji in a comment on this site.
edit: apparently HN is a no fun zone and doesn't allow emojis, but give it a shot and you'll see that they change when you paste them in a new page.
Take the cat from the 9to5 link in the other comments. If I like using the smirking cat, for instance, to indicate potentially devious behavior, I will no longer have that option because they are turning it in to a smiling cat.
It would be possible in editting software, of course, just like fonts, but in messaging (the original and arguable core use of emoji) not so much.