The only appliance that requires the license is your core manager appliance (where the agents connect). Essentially with multiple appliances, you could end up with a "stack" - Manager | Database & Reports | Syslog Collection | nDepth Storage & Search. It's useful to help scale or distribute processing by virtue of separating duties.
This is still supported, but right now requires support to help you get it set up. On v5.5, it won't take long to do.
One thing worth mentioning, we don't recommend sending multiple appliances to the same storage stack because the data ends up inter-mingled and hard to search. Things like Reports make an assumption that all the data in the database corresponds to a single appliance.
Thanks, this is exactly the level of information I was looking for.
Until you read the "Scalability" section in the manual it isn't nearly as obvious how robust and scalable this product is. It's also nice to see that you don't have to license each of the different appliances in your stack separately. This all certainly adds a level of value to the product that isn't initially obvious.