OK, since it was my comment that started this off.
Firstly, I would ask anyone who doesn't understand the problem to walk the Ridgeway section I described. In winter it's a swamp in places, and in summer when the tyre tracks have dried, it's a case of jumping over the holes which can be 2 to 3 feet deep. In many ways it's the lack of self discipline exhibited by the drivers that causes the problem - in some places the Ridgeway at this point is almost wide enough for two vehicles, but no space is left for walkers. I once had to traverse a particularly bad (but very wide) spot by hanging on to the bushes at its side. I would also point out that some of this section (of the Ridgeway) actually has signs up saying 'No Vehicles' which it is obvious are blatantly ignored.
Next, and thankfully it's a seasonal thing, but the first time I went up Pen-y-van (Brecons) was at Easter when apparently there's a track that goes to just below Cribyn which is opened to 4x4s for about 2 weeks. The whole day was marred by the constant droning of petrol and diesel engines as they ground their way up and down the valley. So this complaint isn't so much about the mess they make as their noise pollution. I use this example as
These are two specific examples as they are exceptional, but I have often had walking spoiled by having to negotiate tracks that inconsiderate 4x4 owners have churned up into a bog. I have somewhere a photograph of the track on the approach to Blorenge (nr. Abergavenny) which involved leaving the path altogether it was so messed up. If I can find it I'll post it as an example.
Then there's also the point that these things DO generate quantities of noxious gas that let's face it, walkers don't. My feeling is that the beautiful parts of the countryside that 4x4 owners feel they should have open to them is in danger of becoming as polluted at the towns if they are allowed to run free.
Now I know that the 4x4 argument is that walkers also mess up paths by causing erosion, and I know that there is some truth in that, however one 4x4 can cause the damage in minutes that a regiment of walkers could only do in several years. I also believe that one of the advantages of walking to the nation is that it keeps people physically in good shape (and therefore reducing the strain on the NHS), whereas sitting in a huge lump of noisy metal is hardly good exercise and pumps poison into the air which one day the economy as a whole will have to deal with.
So sorry titaniumdude, but I disagree with your comment 'Lets be honest, it's not that difficult to get away from them', as I think it can be very difficult to get away from them. Unfortunately, it's the wilder parts of the country that do appeal to 4x4 owners because of their wildness and not in spite of it.
S.