Look at how the whole Internet scales. We don't even bat an eye that an average DNS lookup takes less than a second. The SMTP mail system also has proven scalability. Of course some people are already chomping at the bit to write to me and explain what an idiot I am. There are problems with replicating files and HTML across multiple servers.
Some applications are very sensitive to concurrency issues and race conditions.
There are cases where the accessor of a file must have the absolute latest version and must be able to retain a lock on it. There are cases where we want to lock just portions of files, etc. However, these are not the most common cases. For example we could use dynamic DNS and failover scripts to ensure that transparent availability was maintained even through the loss of three of the four servers. There's certainly some robustness to this approach.