As you've probably gathered, the G5RV isn't exactly the 'cure all' of antennas. It does work well on 20 meters for which it was originally designed, but only 'works' on other bands because it is harmonically related. That was never the original intention for that antenna. A tuner is required for reliable use on those harmonically related bands, and for the WARC bands, forget it.
If you have the room to put up a G5RV then you probably have the room for something like a 'fan dipole', which really does 'work' better on it's design bands. All multiband antennas are compromises in some way, there's no getting around that. It doesn't have to be a large compromise, but there's always a reduction of performance on at least one or two of the bands it's supposed to cover. I have to think that having an antenna that will at least 'go' to some none designed for band has to be better than not having an antenna that 'works' on that band, so they are certainly an option (something is better than nothing, right?).
- 'Doc