There's plenty to be gained. On a normal system running an x86 OS with 4 GB of system memory installed, you'll see a max of about 3.2 GB of memory....even less depending on your video card configuration. A recent thread on here had that down to as little as 2.5 GB. I don't know about you, but I would feel as though my money was being wasted if that much of my memory was being wasted. Would you be happy buying a car that advertised 200 hp, but yet you really could only use 120 of it?
Deacon, If you look at the naked numbers, you have a point - there is on average 750MBs of RAM that cannot be used due to the address space problem with 32bit. However, if you look at real life, i.e. the amount of RAM that is really being used by 90% of the users, you will find that these 750MBs are never needed anyhow. Have a look in you Resource Monitor > Memory tab and check how much of your RAM is "in use". Usually a lot less than 4GBs.