That is like saying why do people make private servers or mod a game. You do it because you want to make it your own, you want to make it how you want it, and in some cases you want to make it to make money. If someone made this it wouldn't have to be for just RageZone. It could be the next Skype.
People make private servers for higher rates and easier gameplay. This concept doesn't apply to chat applications
at all, plus it has already been done with XMPP. Waste the time coding a system if you really want, but there are a large variety of different XMPP clients with any features you'll ever need, not to mention the huge array of server-side features.
Even if you do make something worth using, it still wont be the "next skype". It used to be MSN vs. Yahoo messenger. Over time by chance more and more people used MSN. Over time people started using MSN only because their friends used it. Since it merged with Skype, Skype is now the "default go-to" communication method. Everyone has it; its basically a social standard.
You may be able to get users, but the sad fact is you'll never compete on Skype's level for a public chat application. You need users, and you wont get users when no one uses your service. Worded more intelligently; you won't get users because "everyone else" already uses Skype. The only reason people switched to Skype was because it was forced. Otherwise everyone would still be on MSN, because it was "first" and people don't like change.
All that aside, you're going to spend weeks, if not months of development on something that
has already been created and does exactly what you're looking for, so you and your fellow developers can have a better environment to communicate through. Delay development to develop something not worth developing, in an effort to make developing your original application more efficient. Oh the
irony.
You can send files over irc and probably at faster speeds than skype.
Yeah, in a totally annoying way depending on what client you're using. Its also user to user. Skype can send files to everyone in a group, and I'm really not sure why you're hinting at skype transfers being slow. I've never had transfer speed issues via skype.
IRC can't beat dragging and dropping a folder of files to share with an entire group, a process that takes ~3 seconds to start on Skype. Don't get me wrong... I love IRC, but its time to move on for smaller scale communication.