Most people who have tied on a rotary fly tying vice will agree that it has some advantages over the non-rotating fly tying vice.
- You can rotate the fly as you wish to inspect it from all sides
- You can easily turn the fly to get any tie in side facing upwards - not only over or underside of the hook - making it easier to place and secure almost any material.
- You can turn the jaws to better tighten or - more important - untighten the hook holding mechanism which is often hard to grip properly.
Apart from that the rotary vice should easily be able to do whatever a non-rotary fly tying vice can do:
hold the hook in the position that you set it and keep it there, personally we like vices with heavy pedestal bases so that the vice does not move when tying flies. . Some of the true rotary vices can be tipped like many of the non-rotating ones however, once tipped they will then loose the ability to rotate the hook truly, but if you want to get the hook higher or get more space behind the hook, it is achievable this way.
Contrary to what many non-rotating people think, the vice will not rotate when you apply force to the tying thread or materials, indeed it is locked and works exactly as a standard vice. The torque applied so close to the axis of rotation has no effect on the fairly heavy mechanism of the vice unless it's running on ball bearings and the fly is very thick. On some of the special vises with very smooth and easy running rotation mechanisms this might be the case, but on any rotary vice you should just lock the mechanism and completely be able to avoid this problem.