You can reduce the number of threads because you can configure it so that all your virtual servers share the same thread pool.
Presumably the bet is that the odds that all sites will simultaneously reach peak activity is low. If you have four AOLserver instances you need to configure each for max traffic, if you share the pool space you can scrimp.
You can also set up thread pools for the virtual servers separately, and by URL space I think ... it's very flexible.
The only downside is that you can't restart the individual servers individually.
