reviewers run benchmark without cabby cause:
1. hassle free & less time consuming: installing the motherboard, GPU, proccy (along with the HSF), PSU & finally shutting the lid off. it'll take more much time to do the basic assembling.
2. no impact of cabinet: not everyone's cabby same. some may own a highend cabby, some owns some rather crappy cabby. so cooling will be different for everyone's PC. also most reviews are run in AC rooms.
I know the reasons why reviewers most of the time don't use a cabby.
By always I meant to say they don't use a particular high end card like HD5970 24*7 without cabby - they keep on changing mobos, cpu, gfx card in every few hours - so testing a rig without cabby is more comfortable to them.
BTW, nowadays they use some open air atx cabinets to test bench new hardwares.
Cage Match: Four Open-Air ATX Chassis : Better Than No Chassis At All?
I told him to buy a cabinet so that he can secure the gfx card firmly. I've not recommended about any high or low end cabby.