I think O.L said it all! It took the intentional destruction of well over a gross of carbon shafts against a piece of armor plate for me to work out what it took to get the IF's dimensions and degree of progressive flex right. And I'll guarantee that the IF's are tedious enough to make that not many folks would ever make their own!
Outside-sleeving carbon shafts helps on direct-impacts, but it doesn't provide the progressive flex; which is what it takes to reduce shaft damage on angular impacts. A 'graduated-flex' outside sleeve would do that, but it also increases shaft diameter; reducing the shaft/ferrule diameter ratio. It's desirable to keep shaft diameter small, ergo, the reason for footing them internally.
Among the 'normal' carbon shafts I've tested, the Grizzly Stik ranks as most durable. I've not had any problems getting great flight with them, and I tune them the same way I would a parallel shaft.
JC, one of the benefits of the IF is that it shortens the shaft's 'working section', stiffening the dynamic spine. Most of my bows are not anywhere close to center shot (which would need a bit more spine) but I had 60-75 shafts shooting perfect from bows up to 90 pounds (at my draw) with high front-end loads. On one older HH 85#, it had to drop back to a 45-60 shaft and still leave it a tad long, and that was with 465 grains up front (including the IF). One up-side of this IF shaft-stiffening effect is that it allows use of a lighter static spine shaft, which lowers the mass-weight of the shaft's rear a bit, further boosting FOC.
Ed