Actually, the "Beer Can Cold" works like a champ 95% of the time.
The hvac girls are saying that only a hack would use it, but they've probably reached down a few times and grapped the evaporator line on the condenser to see if it was beer can cold and then made a mental note that the system is properly charged. It is so damn easy and so accurate. That's why you should use it first, if you want to know the condition of your system.
Besides, we are not talking about hvac girls here. We are talking about customers. Customers who have been screwed by the professional hvac girls too many times and have seen them on TV hidden cameras screwing other people.
It's pretty simple to use really.