To add to what Bax has already said on this issue. The ferrite ring you refer to serves as a choke and will filter out very high frequencies if they get on the signal line to the servo and prevent them from getting to either the servo or receiver. You are correct in they are very fail proof in that it is all passive parts.

The one I just saw on Radical R/C was called the "ACE" filter. If it is the one which I am familiar, it is an IC that amplifies the servo signal. In amplifying the signal it will also help block any induced signals of high frequency. I keep saying high frequency because if it blocked the low ones it would not pass the servo signal either.

One must keep in mind that these devices were needed more in the past when the receivers had a marginal drive capability. I had a receiver that had to have a buffer/amplifier in order to drive two servos off the same channel. The receivers made today will drive multiple servos without help. BTW, the servos have also improved and they don't need as much drive from the signal.

Do you need a buffer/amplifier or choke????? My answer is maybe, maybe not. One thing is for sure, using them will not hurt you, unless of course they fail. Will they fix a problem? My answer is maybe, maybe not.
You may have a problem and installing a choke appears to have fixed the problem when all along it was a bad connection of the cable that was replaced by the choke.

My suggestion is to see how things work first hooked up normally. If you have a problem, trouble shoot it and if that fails try a buffer/amp/choke.