Yes, Matt, it is a good idea to carry spare inserts for Ortlieb bags, and I do so myself. Loss is generally not a problem with new bags (unless the're not snapped in securely to start with), but can become problematic over time with the effects of wear, rough roads, and heavy loads. There's not a real good solution to the problem once it occurs...users have employed dental floss, glue, and various tapes to keep worn inserts in place, but it is not much fun to search for them when lost. Loss typically occurs when the bags are demounted in camp or at a lodging. This is one reason why some people pad the rack out to such a large diameter they can dispense with inserts and use just the outer hooks, which have no parts to lose.
I also do something else, which accomplishes the first task as well: I have each of my rear bags fitted with a spare pair of hooks and inserts on the rails. It makes unauthorized removal ("theft") of the bags a little more difficult, and provides a handy place to store the spares in case of crash damage or loss. Placing the spares on the more heavily-laden rear bags helps distribute the load through a total of 4 hooks/adapters per bag, rather than two, and the spares take up no space inside my bag. Added weight is negligible.
I also use two lower retention fins on each of my Ortlieb bags. The extra fin makes for a more secure, rattle-free fit and provides a ready spare in the event of breakage (lower retention fins have been known to break after some time, especially in cold weather).
With new bags, you'll be very unlikely to have a problem. With time, insert loss can become a problem for some. I figure I'm covered either way with the spare set of hooks/inserts, which of course fit any of my four bags. All the Ortlieb hooks, adapter inserts, and lower retention fins in the QL-2 mounting system are interchangeable and can fit any location (not side-specific).
Hope this helps.
Best,
Dan.
