Walk-In Cooler Server Room?

After searching a little bit, I wasn't able to find any specific information on this, so here I am. We've got a single 42U rack with 6 servers and some switches. It is currently housed in a coat closet with a 14,000 btu portable air conditioner (our third!). We need to move our servers into a properly configured space. One idea that has popped up is a custom built walk-in cooler. At first, I thought this might be a great idea, but then I thought about ventilation, humidity, and electrical concerns. Does anybody have any thoughts on this idea? I'm guessing that there are some really obvious reasons (that I'm missing) why this is not a good idea...

The idea was actually used for server rooms at one point in time. Today we understand that in-rack (small deployments), hot/cold isles, raise floor ventilation, and the 'newest' open-circulation designs are more efficient and have a considerably better ROI than putting your DC in a commercial size refrigerator.

Why not a custom-built proper server room instead?