If you have a lot of growth and wind up filling up an entire system, most products on the market today don't have the ability to add another system behind it. So, you have to invest in a new system. And that leads to another problem. If you get a system that's the same size, you can't simply add the previously deduplicated data to the new system, you have to start the whole deduplication process all over again. If you spend the money to get a much bigger appliance, you can migrate all the data over and start building from there.
At the enterprise level, you are looking for a product that can scale performance and storage independently.
Check out the entire Data Deduplication FAQ.