No, mostly because it's inconvenient. Making the root filesystem read only means having to track down everything that tries to write to it and fix mounts for them.

After a few years of working with this, I kind of wish we'd bitten that bullet at the start. The AMIs we produce have a root filesystem that's only like 2 gigs so it goes across the network faster. 2 gigs isn't a lot of space when something decides it wants to log or write out temp files in weird places.