One of the main asset of the Business Delegate design pattern is to : "hide the underlying implementation details of the business service, such as lookup and access details of the EJB architecture." (Sun's Core J2EE Patterns - Business Delegate)
In other
word eveytime you need to do a lookup to an EJB you might consider using a business delegate.
In which tier are you doing lookups to EJBs? Well in both the web and the business tiers.
For instance in the web tier you may have to access some EJB session beans using the Session Facade design pattern.
In the business tier the EJB session bean may have to access some EJB entity beans.
In both cases you need to use some lookup mechanism so the Business Delegate design pattern could be used in both tiers.
Since this pattern hides the "remoteness" of accessing EJBs it might be less advisable to use it in the web tier
without using a coarse grained access strategy to the EJBs (like the Session Facade design pattern).
Due to the distributed nature of EJBs you still need to use a lookup mechanism in the business tier because the deployment of the EJBs might be located on many JVMs / machines
so the Business Delegate design pattern can still be useful in the business tier.
Regards