Well, it'll work, but according to RusselB, and I assisted someone at the forum awhile back, and I quote, "If you don't use the ! in front of the command in these circumstances, mIRC will just keep calling the alias rather than the internal command." That is to say, if your alias command is the same as mirc's default one. You can reference to this post below and you'll see what I mean:
http://www.hawkee.com/phpBB2/viewtopic.php?p=109349&highlight=#109349
/tnick is a command; it's not an identifier. MoshMage, I didn't make the rule; it's part of mIRC's coding grammar.