Also, when you left holes open like MikeC does, what about electromagnetic emissions? PCs were designed to be run in closed, metal cases. That dampens electromagnetic emissions a lot.
Think about all the dual-fanned and 120mm PSU's on the market today. There's a big hole at the bottom where all that nasty EMI can get out of the PSU and zap the rest of the PC. Apparently, though, the EMI doesn't seem to be much of a factor after all!
If your computer is in a lab with sensitive equipment, then EMI might be something to consider when you've got the dremel in your hand and you're ready to make Swiss cheese of your case. But for PC's at home or at the office, I just haven't heard much fuss made about EMI emissions.
That's because there's a grille on the 120mm fan. EMI can be (and is) blocked by a grille. Sorry, but your argument is invalid.