pros and cons of installing biztalk and sql server on same boxes

[btsuser]

Hi!

According to ms web link:
http://msdn.microsoft.com/en-us/library/aa578057%28BTS.10%29.aspx

"The smallest BizTalk Server deployment that provides high availability for
the both SQL Server and BizTalk Server is made up of two computers that have
an active/active cluster configuration for SQL Server. Both computers contain
instances of all the BizTalk Hosts in the environment. If one computer fails
or encounters errors, the other computer maintains service availability for
both SQL Server and BizTalk Server."

What're the pros and cons of this configuration compared to a 4 box
configuration (2 biztalk server boxes and 2 clustered sql server boxes)...
except cost considerations ofcourse :)

thanx.