Are they necessary?
You may have seen those new link belts that replace the standard v-belt on your woodworking machines. Here's my opinion on link belts.
Links belts definitely make stationary tools vibrate less, but I believe that the need for them is determined by the type and quality of the machine. Obviously the more the tool vibrates using regular v-belts the more you are going to notice the improvement.
Some machines will vibrate more than others, based on their design, operating speeds and quality of manufacturing. A link belt will reduce the vibration caused by the orignal v-belt, but won't solve other issues that cause vibration, like off-center or bent arbors.
The reason primary that regular v-belts vibrate is because they get "out of round" or develop "memory". A belt will not vibrate very much if it kept in a perfect circlular shape. But stretch it between two pulleys and it becomes oblong. Leave it motionless between those two pullys and it it want to stay in that oblong shape. Turn the tool on after leaving it in that oblong shape for a while and you'll get a slight bump everytime the belt goes round that spot again. So the solution is to never stop the machine, OR somehow minimize the "memory" effect.
If you're going to store the machine for an extended period, loosing the belt or removing it is a good idea. The worst offenders are the belts on that old saw that's been sitting unused in garage for the last 10 years. Belts that are too tight will also be like that. Contractor's saws are prone to that because of the way the motor pivots in the back and is suspended by the belt.
Because link belts can flex, they don't suffer from the "memory" effect like regular v-belts. That's one of the main reasons they are so smooth and reduce vibration.
My drill press doesn't vibrate noticably at the slower speeds but did at the higher speeds. Now with the link belt it's much smoother at all speeds. My old (old as in I don't have it any longer, not age) Delta contractors saw vibrated a little so I tried the link belts and was very happy with the results. By contrast, my Delta Unisaw with 3 regular v-belts is much smoother than the contractor's saw ever was, even with the link belts. I'm not sure how much benefit I'd get placing a link belt of that.
Interestingly enough, I've discovered that I've haven't felt the need to replace the v-belts on any of my USA made Delta machines. These machines come with a much high quality ribbed v-belt which is almost as good as these link belts.
One thing to remember when you first install a link belt is that they are stiff and noisy. For those northerners who remember the days before all-season radial tires, new link belts sound a little like driving on a dry pavement road with winter tires on. Some people are surprised to hear more noise when they first switch, but the noise goes away as the link belt becomes more flexible with use.
One note: many people get confused between "Fenner" link-belts and "Power-Twist" link belts. In fact they are the same thing, the Fenner company makes the Power-Twist belt.