|This is the "standard" set-up.|
On the standard set-up, experienced players don't have to think about where to build. They play out the same strategies game after game. The same spots are going to be good starting positions time and time again. Not so with randomization. Randomizing the board makes the players question even their initial placements.
A fan of the standard set-up might say, "But randomizing can create stupid set-ups, such as the desert, 12, and 2 being next to one another or two eights and a six meeting at one intersection." To this, I would say that my experience has proven this to not be a problem. If you've got a "hot spot," the competition in this area will be fierce. These battles for control can often be their own little mini-games with people cutting each other off and building just to block their neighbor. These hot-spots also tend to attract the thief quite a bit as well, limiting their upside. Meanwhile, probability-dead zones allow for other players to expand unhindered, perhaps building a long highway into the wilderness to take the Longest Road.
Another thing that standardization fans might say is, "Randomizing the board doesn't allow for balanced resource creation. All of the high probability numbers might end up on one resource while another isn't generated at all." This is, of course, true, but I don't see this as a negative. When these things happen, it increases the value of ports, encouraging players to build toward the edges of the board. If tons of ore are being produced, then you can bet it'll be a foot race to the ore port. Other players may wish to quickly build on a 3:1 port just so they can get some of a resource that is in short supply. Even in a standard set-up I've usually found this happens anyway because the dice don't perfectly mirror probability. Any experienced Settlers player have regularly seen games when, for example, 8 isn't rolled at all and 4 is rolled twenty times.
So in conclusion, randomization is the way to go when setting up Settlers of Catan. It adds great replayability to the game and poses unique challenges to experienced players.