In reply to MischaHY:
I thought I'd pitch in here, simply because it's an interesting discussion.
When it comes to skinny I almost think there's two tiers: there's the long/skinny ropes you're likely to use in Europe and there's the short/skinny ropes you're likely to use in the UK.
For the former, you'd realistically be expecting these to be in/around the 8.9-9.2mm marker. When it comes to falls/durability, because you're on longer pitches (i.e. >30-40m+) if you do fall it's likely to have quite a low impact force because there's a whole lot of rope out - as such it won't wear as quickly.
For the latter, I would say these are likely to be in.around the 9.2-9.5mm marker. If you're climbing routes places such as the Catwalk at Malham, LPT, or the Cornice then the routes are short + hard - hence you're likely to accelerate the wear dramatically. I must admit that I'm still using a 10mm for routes of this nature because of this: anything skinnier wears far too quickly.
Finally, in the eyes of your average climber I still think that the vast majority are buying 10mm singles and 8.5mm half ropes; however, this is changing and I would expect - were the sales research ever to be made avaialble - that the diameter has decreased (i.e. from 10.5mm and 9mm respectively), and will continue to decrease to something nearer 9.5-9.8mm and 8.2-8.3mm, over time.
Either way, I can see where Theo is coming from.