heating pads- to be or not to be

Question
I have 3 leopard geckos in a 40 gallon tank who have been doing well.  I have always (4 years) heatet them with an overhead bulb but in my downtime research..since you can never know too much about your pet...I've been seeing a few people say you NEED a UTH to help with digestion.  Unfortunately, i have excavator clay by zoomed as there substrate that says not to do that.  Another expert on here..jennifer i believe..said you don't need a UTH but if you want, to put it on the side of the tank.  I'm just a little confused now since I was on the brink of getting one and now I read this...I really respect what you guys say after reading your answers and seeing how knowledgable you really are.  I'm introducing new roaches into their diet and was thinking that if UTH's really do aid digestion like that then there's no reason for me not to get one. Just wondering what you think.

Answer
To be honest with you, if you already are using excavator substrate then don't use the UTH.  I have a free Leopard Gecko Mini eBook on my site which you can download.  Regardless, UTH does help with digestion, is it totally necessary? Not all they have been doing well for 4 years without it so I wouldn't worry about introducing one now.  If you were interested in breeding I might look at one but if the ambient and basking temps are where they should be then I wouldn't change anything.