I agree. If you are a PC builder and use the computer to test components then I would keep the case open but close at end of use.
If you are using fan cooling I advise not to keep open side panel.
Dust is an important factor but fan cooling requires air flow.
In from the front, out the rear; constant air flow to carry out heat.
Having the side panel open does not move the air flow through your computer.
Now if you are playing Half Life II and overclocking, your computer will get hot.
Without having extreme air flow and I mean extreme, your game will stop (maybe your computer too). A lot of gamers who don't have huge setup's will remove the side panel and put a box fan next to the computer.
This approach is called forced air and it works. You are forcing air across the computer which removes heat. The downside of this is: noisy, could add extra dust, and just plain ugly!
As a beginner gamer, I have tried to create a natural fan/heat-sink cooling procedure to keep my computer cooled at all times.
12 fans later, I think I have done it.
Hope this helps.
Your Hardware Guide.