A reason for 64-bit Linux
Linux has been 64-bit since the Alpha port
back in 1994. Despite this, I have very little experience with 64-bit Linux, as I've rarely seen a need for it in what I do. We're finally hitting the 4GB limit of 32-bit machines, so this is starting to come up. However, just today at work
I found a more pressing need for 64-bit Linux: The per-process RAM limit.
32-bit Linux processes have a theoretical max of 3GB. From recent experience, things actually start to get funky at about the 2.5GB mark. One of our research labs makes heavy use of the network simulator NS2. Unlike many applications, it seems to be single-threaded. As a result, one of the grad students here hit the 2.5GB problem spot in under 30 seconds. The solution: 64-bit Linux.
It took me about an hour to reload and restore all user accounts/files from a 32-bit Ubuntu install to a 64-bit one. While there are other people using 64-bit Linux in the department, this was the fist time that I saw first-hand a real case where it was required. Nifty.