There are many that dont have one, but what you need to determine, is what kind of correction work are the guys doing? Is it on very lightly swirled paint or really badly scratched, rids, and swirls, etc., that always require compounding and/or polishing, etc..
If you intend to sometime Detail high-end cars, all the expensive German, Italian, English, etc., vehicles, then it would be a very good idea to have that PTG to help you be sure you knew what you were getting into, as you were figuring out your plan.
I always bring my Paint thickness gauge out with me whenever a prospective Client comes by or I go look at a vehicle, and with the Client, I go over their paintwork and advise them how much material is available to work with and this helps us formulate what they may want to have done. Do they want just a clean up of the damaged paintwork or perhaps spend a few more hours and get it really looking clear and deep but not perfect, and there is still lots of material left on the car for future work?
This valuable tool has helped me numerous times and totally helped convince the Client that I know what I am doing and I am willing to help them better understand what is going to happen, and what condition their paintwork is in - on the top and all the way down. Most people love this stuff.
You can spend a few hundred or a few thousand on a PTG. You have to decide what the ROI will be based on a number of things, that are going to be particular to your business plan.
The best PTG's will show you pretty well all the different layers of material, and do print outs and stuff, and the most basic will give you the total thickness in Mils and Microns, and work on steel and aluminum but not plastics or fiberglass.
I would advise anyone to at least think about getting a more basic one to start, like the Highline meter, or a similar one. Any good meter will be able to give you some peace of mind and be way more accurate on what is on ever inch of a panel rather than trying to guess or asking the Client if they can remember if it has ever been Detailed before, etc..
Good luck with your research !
Dan F