The short answer is that you don’t need to use Reverse Osmosis ahead of a DI system… unless you prefer to spend a lot more money than you need. Then by all means, just use DI. In fact, there are many companies out there who’s revenue streams count on you not using Reverse Osmosis.
More often than not, when a company installs a commercial reverse osmosis system, they start with a system that really doesn’t fit their needs for a variety of reasons. Most times, the reason is that they simply don’t know what they don’t know.