The reasons that I just stick them on with a couple of pieces of aluminum tape instead of the adhesive is from lessons that I learned early on.
1) It's easy to remove and re-apply whenever you need to wash and rinse the enclosure out. You don't have to mess with the cord getting in the way or accidentally damaging the connections. Also, if you upgrade to a new enclosure, you may be able to use the same UTH, and it makes transferring it to the new enclosure quicker and easier.
2) If you accidentally drop a rock hide through the bottom of the tank, it is alot easier to peel off two pieces of aluminum tape in order to put the UTH on the new tank than it is to try and seperate the UTH from all of the pieces of broken glass stuck to the adhesive.