Read the full page.
http://www.karinya.net/g3txq/chokes/
Steve G3TXQ has done a lot of experimentation into baluns, ununs and common mode RF. The chart I posted is one he did using proper measurement techniques and equipment. As far as I know he's the only one who has published such data.
It is a necessity. Whenever you're using an unbalanced feeder, aka coax, you need one to prevent the coax becoming part of the antenna even when you're using a perfectly balanced dipole antenna.
The problem with it is that in many cases there will be no noticeable difference to the end user so they think that its doing nothing because noise on RX and RFI is the only measurement they use and that was the same after they installed a RF choke as before they did. They don't ever measure common mode on the coax and, in the case of balanced antennas like yagis and dipoles, the radiation pattern as it is altered by not having a RF choke and the coax becoming part of the antenna.
Without a RF choke throw 100W into your antenna, get a small flourescent tube and place it near the coax at various points and you'll see it light to various degrees with no other source of power. That is proof of the existence of common mode. Fit your RF choke at the feedpoint and repeat the experiment. Depending on the effectiveness of the choking and assuming its been built properly you'll see either the tube not lighting at all or lighting far dimmer thus proving the choking is working.
If it helps, think of using a RF choke as "best practice". Do it for the same reasons you don't just take your coax, strip back the insulation and just shove the wire into the antenna socket, holding it in place with matches and insulation tape and instead solder a PL259 on the end even though the other way works.
