Biztalk Server 2004 follows a Hub-Bus hybrid model and why it is a better model is because it takes the best of each of these individual styles namely Scaling out , throughput and golden copy of data( Centralized data store ) . Every message gets tored in the messagebox . This is how Biztalk achieves Statelessness. This is one of the reason why the Debugging tool ( HAT) is like the way it is .
Although I have heard lot of people complaining about HAT and comparing them to Visual studio debugging support which are excellent many dont realise the nature of Biztalk . Unlike traditional debugging enterprise Stateless application like Biztalk being stateless in nature writes out events to a Database . and a tool like Kat is just a reporting screen of those events . ( You can do live debuggging -but only if you explicitly set it to)
Another way to get to see errors in Biztalk is to run Debugview from
www.sysinternals.com . Now you can write out your Debug messages using System.Debug Namespace