While this is true, the majority of us bassheads are always changing equipment whether it be amps, head units, processor, etc..... Every time you change out that equipment the gain has to be adjusted again. Also you can ( like someone suggested ), charge a fee for other people in your area to help them out and set gains.
I Just recently got into the audio world, since purchasing my DD-1, like 3 months ago, have used it to set my gains on two HU, three amps (my equipment), cousins HU two amps, co-workers HU, Crossover, three amps. Trust me having this tool is not a one time use deal. It has paid for itself and more in the short time I've had it. Absolutely don't regret buying it.
What you can do is register yourself in the tools map to try to help others and maybe charge some type of payment..
You can help your friends set their gains correctly, and I'm almost 100% your not going to have one amp in your life time..
Dont get me wrong guys, i'd love me some SMDamore goodies!
But im not going to run around town settings gains for people lol
I have a descent scope at home, and i use it for more than just gain setting, otherwise i would recommend the DD to anyone who doesnt know how to use a scope.