Other than putting one heck of a load on your alternator by asking it to do something it was not designed for, I don't think you are getting much accomplished. As the previous poster mentioned, @ idle not much charging is going on. I guess that in an emergency situation or in a pinch, it would be decent. But for something like this to be employed as habit, you are asking a lot from your truck's electrical system.
Also, someone who knows cars more than me might contend that connecting and disconnecting dissimilar electrical circuits is bad for the truck's computers. I don't know how much more robust computers in vehicles are now when compared to one 10 years ago, I do know that in the past, it was always frowned upon from the mechanics that I spoke with.
