Guys can say what they want about the window, but this is from my experience.
Two years ago my son and I were cleaning, as with any young kid most of the time they are faster than lightning. The very first job he did with the window he was completely shocked. He went in to wand a room and everything was going as normal....this was your normal BROWN apartment nylon type carpet as he was wanding out of the room he hit the traffic lane and the window turned dark with all the dirty rinse, and he did a second pass, and a third, he did it till the wand window was showing him that it was good to move on.
He looked at me and said now I know why you take a lifetime to clean, MOST VET or BETTER cleaners just put two n two together and know that curtain areas of the carpet are just going to need to be flushed better, this is why some carpet cleaners just have a good rep in the area, just better cleaning skills and knowledge....even with out the widow.
But my Son said Dad that is killer because I would have just went over it like all the rest of the carpet and I would have HONESTLY thought I did a good job cause I didn't know.
I personally value the window, on poly carpets the first pull is dirty maybe even the second, but the POLY carpet still looks like crap....the window doesn't lie. I have proven it to myself over and over, I have even pulled out my roto vac to see if I can get it better, 8 outta 10 times the wand and good chems has pulled as much out as possible.... The POLY carpet is crap and it isn't gonna change anymore.
I honestly believe the window is the most valuable thing when training a new person, you can show them, hey this much moisture coming past the window is acceptable and explain you don't need to spend any MORE time than that, or just the opposite and say this much moisture is to much to be left behind.
I will say that the window without a doubt has helped me more than it has cost me.