I just went to check, all of the 8mm shank festool bits out of the caroussel set have solid shafts.
I remember that, what I thought was a 5mm straight bit, I bought per accident once, had a hollow/threaded shaft, and that bit turned out to be for boring the 5 mm holes for the lr32 system (I wanted to check that one, but I no longer have it (it probably was in my of1010 systainer when it got stolen).
I just found out that one of my most used festool router bits also has a hollow/threaded shaft, it's an 18 mm straight bit with reversible/replacable bottom and side inserts. I don't think it's significantly less strong, because it's only the top 8mm that is hollow, and it's supposed to be inserted at least 25 mm into the collet, according to the markings on the shaft.
I thought that on the 5 mm bit, it was for mounting on different machine, like the bits for the domino mount, but then on a doweling machine; but I think that on the bigger bit it was part of the production process of the bit itself. to temporarily mount it for machining the sides/tapping the screwholes/whatever. (I'm fairly clueless on how those bits are made, just my imagination
)
/edit: btw: I also checked the festool 45° bit for making folding miters on plasterboard, it has a solid shaft.
