I sat out the last round at Cata3 because my back was killing me (no longer getting exhausted from a full day of Nerf - thanks Gym membership!), and having access to some of these would let me do a single dart sweep between rounds with my magnet wand, fill them back up, and then do very little ground-scrounging during the rounds. Less bending = more Nerfing for me - at least until I get rid of my paunch and strengthen my back muscles.
I know for a fact these would help me, as I used three of BabyGhandi's jello-shot loaders, and when they were loaded and ready, they really sped up my loading and got be back into action that much quicker. What is most important is a design of shingle that will let you carry a decent amount of foam without hampering movement too much. I like what I am seeing so far, but I am thinking something more along the lines of the elastic-banded loader holders they use in paintball would be more up my ally. (i.e. something softer and more flexible for holding the tubes, so as to decrease weight and increase mobility)
I should say: don't get discouraged just because Zorn, Venom and I shit all over your idea. We have rather strong opinions of what is and isn't war-worthy, but we're just voices on the internet. Whether people use these or not is the true measure of their value.
To add to this, I would say that most Nerfers agree that unmodded Stampedes aren't really "war-worthy", but I have now been to at least two wars where said unmodded Stampedes have contributed significantly to the success of their team in at least a round or two. (The first time for scoring numerous points in a Defend-the-Core round, the second time used dual-wielded with 35-round drum clips in defense of the bucket in a Carpe round.) "War-worth" does not always equal "fun".