Yes I have bought such FARES, but not used it to the extent quoted. Even though such a fare may be referred to as "fully flexible" there are still restrictions such as HIP (noted by
Kiwi Flyer) and MPM (Maximum Permitted Mileage).
So the real trick is not to purchase the fare say from London to Sydney, which would be very expensive (on BA that would be 4346 GBP), but buy it from AMS to SYD, which on BA still routes through LHR and costs 2896 GBP. Just need to ensure HIP does not come into play, but from my reading of this on other forums a stopover at LHR should not be a problem. Nor stopovers at intermediate places so long as you do not exceed the MPM.
The next thing to do is ensure your fare in not an airline or alliance specific fare. The quoted routing from London to Athens could not be achieved on a single airline as no airline flies that route. So you need the full IATA fare that is valid on all IATA carriers. These can be more difficult to locate, but worth the effort. Our own Multi-* DSN can provide more details

.
The money saved can be used to get you to AMS in the first place, assuming the starting point is London.
This is quite common in Europe.