I am finding that different knitting sheath designs work with different length needles.
Some sheaths use long steel needles and utilize the flex of the steel needle for the knitting motion. Other sheath designs provide a limited motion pivot point that allows the knitting motion. Such pivot point sheaths can be used with knitting needles that have less spring, such as those ubiquitous 7.5" aluminum DPN (Or, your fine new rosewoods?) I actually have a cashmere pillbox hat going on a pair of US # 1 by 7.5" aluminum DPN using a sheath.
Other sheaths were designed to use very short, curved DPN. (for gloves? baby clothes?)
In any case: Sheaths really do tame the nasty spider aspect of long, sharp DPN. With a sheath, even 5 - 12 inch long #000 DPN becomes more like a friendly house cat than lap full of angry porcupines.
My gansey is on 5 - 18" # 1 steel DPN, and that system works very fast. I really like the way the flex of the long steel needles speeds the knitting. My next pair of socks will be on 12" steel DPN with a sheath. It goes much faster than the pivot point design sheaths. But, it can be very clumsy when trying to fix mistakes.
Sheaths can relive the stress on wrist/hand from knitting, while increasing the stresses on the back. (Not sure if this is inherent in the use of a sheath or just my new knitting chair : 0 )
You can knit *much* tighter using DPN and a sheath than you can with a cable needle. If you want to knit windproof fabric, forget the cable needles. For one thing sheaths go faster, which becomes important when you making lots of tiny, tight stitches.
And, No, I am not going to needles smaller than #000 (1.5 mm) in the near future regardless of what size needles your sheaths were designed for.
Aaron