We do too (Well main services) DNS are on 3 other servers. Backups are on multiple other servers. I can see the benefits of having a main Customer Server Cluster with separate SQL Cluster for SQL Databases if servers are in same datacentre with a 1 - 10Gbps private link between them. The lag could be the difference between sale or no sale. That's one thing. Mail clusters can be a good thing as well I think.