Steven,
I can see where you're coming from and asked the same questions when I was starting with BizTalk. I think the crucial question here is 'how long will a message take to go end-to-end'.
In our BizTalk B2B environment, messages will go from receive to send in under half a second. Furthermore, we take an archive of all messages before the go through BizTalk, so if we lost the databases/database server we won't be losing 'much' – we would need to re-create the environment (and I'd much rather do that from backups than have to re-deploy all of the various artifacts) and then re-submit any messages that were lost. We budget on about a day to restore the production environment (and about an hour to get BizTalk back up and running on a secondary, standby SQL Server).
However, if you have a long running business process that requires input from several systems/users and/or correlation and those transactions were lost, those business processes would need to be recovered from a backup. Period.
So, depending on the length of the business process and the time required to restore the environment (read artifacts) you may or may not need backups. However, I think it is just makes good business sense to have backups and to test them on a monthly basis – I'd hate to be the one to have to tell my boss 'erm…. we don't have any BizTalk backups'….
Nick.