The reason controllers in the US want you to do visuals is that it keeps the movement rate up. There was a long article in AW&ST a couple of months ago about slot constraint into ORD. Amongst other interesting information were the published maximum movement rates under IFR and VFR conditions.
Basically when aircraft were doing visuals the movement rates were about 5% higher.
I still can't understand some peoples distrust of visuals into US airports. Sure, if you are going into LAX on parallel approaches on a hazy day, then no way. But on a gin clear day into BWI or PIT? Keep the A/P in until you have a full picture of where everyone is then go ahead. If it keeps the ATC workload low, we all benefit. (IMHO)