OK, but here's a sorta related question: if someone here asks how they can do something, and they could do it by adding a few lines to a template but it involves using $_GET['do'] in a condition, should they be told to write a plugin because it's the "right" way to do it?
Same thing can be said about any coding standard. Indent vs. not indenting, proper variable, constant, function naming, etc.,.
It all comes down to the purpose and hassle. I don't recommend editing the default register template, but instead finding the do branch in a plugin then calling your own template: That way it's easier to manage and upgrade.
But if he simply wants to add a few lines and the whole situation would indeed be a lot easier to just do it in the template then by all means do it even if vB coding standards says not to. If he was making a mod to be released to vB.org then I'd say he should do it the right way.