Probes placed in the enclosure are left to the mercy of the enclosure's inhabitant. The last thing you need is your probe getting displaced by the animal you are trying to heat properly.
In addition, the manufacturer of my rack system specifically states that the probe should be placed directly on the heat tape.
Since properly regulated heat tape usually doesn’t get much hotter than 110 degrees on the high side, there is no reason not to mount the probe there. It allows the T-Stat to directly control the item that is producing the heat.
I hope this helps to answer your questions.