The purpose of swap (on a modern system, anyway) is to ensure that if a process does get carried away, it degrades gracefully rather than hitting a brick wall. Sure, if you can prove you'll never need more than X amount of memory -- if you're effectively building a very large "embedded system" -- then you can just buy that much RAM and walk away. But for most boxes -- development machines, for instance -- you can't make such cast-iron guarantees.

Really the amount of swap you need depends not on how much RAM you have, but on how fast your CPU and disk are -- the Mac Pro can get itself half a gig into swap before I notice it's happened. But because it's hard to work that up into a rule of thumb ("five times the result of hdparm -tT plus a byte per four CPU Hz"?), and because CPU speeds, disk speeds, and RAM sizes all proceed vaguely upwards together, the rough estimate of needing an amount of swap 1-2x the RAM remains useful and sensible, even if not for the same reasons it was originally promoted.

Peter