The placement of the probe is very important - it's what sets your thermostat up for success. If it's not placed correctly, the thermostat doesn't have a snowball's chance to work properly.
I see what you're saying, but I disagree. If my heating element is centered on the left side of my cage, and my probe is on the left side of the cage, but is in the corner, so it is out of the way, I can set my thermostat to something like 87 degrees, but the actual hot spot, which is what I care about, is reading 92; that's why I say it doesn't really matter.