If I've got a 4gig machine, and running 5 apps takes up 3.8 gig, trying to run another app with necessitate swapping/paging some of the existing apps around to accommodate my new app. If those initial 5 apps only took up, say, 2.8g, I'd have less chance of hitting 'low-memory' situations in the first place.

I say this as someone with 16g mbp and SSD who still hits beach balls and unexplained pauses on a daily basis. Fewer than I used to, but it gets annoying. Not quite as annoying as win95/2k blue screens of 12+ years ago, but getting my ire up.