so come on having reveresed the question without having answered it, lets have three good reasons for using any other form of navigation
As previously stated, I am not having a go at GPS. Your comments are all valid. I am no Luddite and use GPS myself when required (and available). I do however feel it is too easy to rely on GPS. Having the skills to use all the other options (VOR, NDB, RNAV, etc) make for a better understanding of how to use GPS effectively, and the environment you are operating in (ie poor vis, as per the original discussion).
Lille - although of course I did make the point that my comments excluded approaches.
What's that saying again about anyone can take a plane off the ground. You cannot exclude approaches. If you are flying in poor weather conditions you may well need one.
How many know that the standard of their ILS approach for the IMC may have secured a "pass" but would have not done so for an IR?
So how do you expect someone with 15hrs training to achieve the same standard as someone with 55 hrs (for PPL IR). If well trained and current an IMC holder should be able to perform a perfectly satisfactory ILS (ie within half scale glideslope/localiser) down to the CAA recommended minima. I know the minima are different, but the CAA recommendations are on safety grounds and I believe them to be valid, due to the difference in amount of training between the two ratings. I agree that you do not stop learning, and made that point regarding currency. Once an IMC holder has learnt more, then they can think about lowering their minima.
As an aside, the legality of an approach depends on RVR, not cloud levels.