AOLserver is a web application server that powers some of the busiest websites used in the world. Ever used anything run by AOL? In that case you've used AOLserver. Even their services that run on other platforms ultimately talk to an AOLserver instance some where along the way. MapQuest is powered by this piece of software. It's multithreaded, Tcl-enabled, and has a great API for doing everything from sockets, to threads, and one of the best database interfaces known to man. Seriously, once you use AOLserver, you'll hate it because you'll be pissed that you've spent most of your life fighting with other so-called "solution". It makes web development so easy it's boring.
So, why are we using a WEB application server for a mud? As mentioned, AOLserver has all the power of Tcl and then some. Writing a multi-threaded non-HTTP protocol implementation on top of AOLserver is a small matter of programming. We get database connection pooling and such for free. You should be using AOLserver and Tcl for everything. If you don't, you're an idiot or lazy or not powerful enough to get others to see the light.
If it's so great, then why isn't this site running it? Laziness. I just didn't feel like getting php working under AOLserver. Until that time we'll just have to suffer with Apache.