The only problem could be a foam insert can hold moisture causing the bits to rust at the shank. Apparently somewhat common in high humidity areas. If humidity is not a problem in your garage or at a job site, then go with the systainer foam insert as it is efficient at holding bits. In many ways making your own router bit insert for a systainer could be beneficial. These will hold the bits firmly but do add expense.
http://www.leevalley.com/en/wood/page.aspx?p=50697&cat=1,46168,46180
Myself I have a smaller wooden router bit case that slides next to my router in the sys-4. The rest of the 1/2" bits are loose inside the systainer though still in their plastic cases. My smaller non Festool routers are in a Ridgid box, which also holds the smaller bits. Someday I might put together a systainer with all of my bits, but for now it is one less systainer. Combining is a good thing!
Not sure who to give photo credits to, but thanks for the ideas!
I like how they cut up the systainer and added double lays. I would just make a removable tray myself, but this is definitely thinking outside the box