There's two types of weight weenies: (1) those for who weight is everything. (2) performance weenies who place a high value on the advantage of lightness. I think this succeeds wonderfully at #1 but for #2, I agree it falls a bit short. For one, those pulleys look like bushings
rather than ball-bearing based, and that adds significant friction if so. Second, I agree the rigidness may be a concern: it the pulleys come out of alignment due to lateral loads then it will twist the chain which may contribute to drivetrain loss.
When I contemplate getting a Dremel and taking it to my Red derailleur, which has very good (perhaps not the best available... watching those Tiso bearings spin @ NAHBS was truly impressive) pulleys, I wonder if the decrease in rigidity will offset the 10 grams or whatever saved. With many parts I can justify decreased strength or stiffness based on my body mass which is well below the design point for mass-market equipment, but the derailleur load is mass-independent.
It would be interesting to send this to Friction Facts, which from what I can tell does the best testing out there right now.