Although this has a purpose of making comments faster and easier, it has its downfalls.
For example, there might be multiple users who have their usernames starting with the same few letters. This can mean that you basically have to type out the entire name anyway.
Also, I'm not sure about this, but it might be expensive, one, and it would definitely be a little bit of trouble for the team. I am not sure exactly how much trouble, but it might be enough to create a bit of a hue and cry.
so, this is a good suggestion, and possibly implemented, if the team wishes too, but otherwise, why not just type out the entire name manually? It's simple, and it's not that difficult.:)