Bitflipper has a valid point. To me, two of the best arguments for using amp sims are:
1)when you can't turn up too loud. (live on a residential street in Anytown USA and inspiration strikes at 1:00 AM)
2)when you can't turn the devices/ people in your house down. (If only the furnace or washing machine had a volume knob, or my kids for that matter).
My favorite amp right now is a Musicman HD130 that I pretty much use as a tube power amp with a Hafler T2 preamp, an Alesis quadraverb and hushIIcx sitting in a rack with it all running through a no name 4x12 with emminence legends. When the stars are alligned it is the best sounding thing in the world. All the cool hot rodded reasons you would pick rack gear in the first place. The problem is that this makes for that many more pieces patched together with jacks and plugs. I have yet to get an intermittent short that drops my volume 10dB, thus ruining that one perfect take that was destined to transport me into the stratosphere of greatest of the great from an amp sim. (that's how it happened, and I'm sticking to my story.) Plus that thing all told weighs about 100 lbs. and requires 3 trips to get it all from your car to the studio.