Gibbs sliding in from backward point? And don't say that you're not stats obsessed when you're asking me to put a "value" on a subjective argument. Every drop is circumstantial, it depends on who they are dropping and when. There's no difference in the difficult of catching an edge off Donald Bradman or Chris Martin, but the effect of it is far different.
I haven't seen massive amounts of Amla in the slips, Kallis has always been pretty good, I'd compare him to a Warne, safe pair of hands without being spectacular.
I think it's very valuable having a high class slip cordon, it does win games of cricket. At Test match level, you can't afford to give chances. It's certainly a factor when selecting between two players if one can field in the slips, and it also makes it a lot harder to drop a guy if he's a very good slip fielder, even more than someone who is a good ring fielder.
Australia's cordon in the mid-1990s (Taylor, M. Waugh, Warne, S. Waugh at gully) was one of the reasons why they started to emerge as a dominant force when, on paper, their side wasn't that much greater than those they competed with. It's also why they struggled on the subcontinent to some extent, they lost a comparative advantage as not as many catches would go to slips on the lower and slower wickets.
