Personally I believe the foam gun to be useless in the area of detailing. It's a fad, almost in the same form as some rapper wearing baggy pants and then everyone else seeing that feels or believes to be just as cool and hip they also need to wear their pants baggy.
This though is my opinion, which doesn't count for much because like butt holes, everyone has one....even though this can be argued.
So to get to the real meat of this we need to ask if the desire of a "foam gun" is to cover the vehicle in "foam" with the thought that the foam is acting as a pre-cleaner then just how effective is "foam" as a surfactant?
In other words, is the cleaning power within the foam or rather is foam (suds) the by-product of the car wash formula?
Suds, in and of themselves, do no cleaning. Suds are added for the eyes and piece of mind of the consumer because we associate suds with cleaning but it's not suds that clean but rather the surfactants. A cleaner leaning more on the alkaline side can literally have no suds yet be a very effective cleaner. So bathing your car in a thick sheet of foam has only a mental benefit.....and it may look kinda cool.