Sorry Damian, but I have to disagree - I have never yet stayed on a site that required breathables.
Neither Club requires them.
Breathables have these aspects;
1. A few sites require them.
2. They are more pleasant underfoot - particularly on hardstanding.
3. They do kill the grass more slowly.
4. In wet weather you can find water, then mud, will come up through them.
5. They are dearer, heavier, and bulkier.
6. They are more difficult to clean the mud off the underneath.
On the whole I prefer a breathable, but I still carry a cheap impervious in case of really bad conditions.