The glass in the carrier becomes part of the optical system. Although it is not in focus, irregularities do affect the image. This is why AN glass can be such a problem. The best quality stuff seems to work decently, but there is a lot of inferior stuff out there that definitely degrades the image. I've generally managed to do without it. For the plain glass, window glass is out. Good quality float glass seems to be OK, but it is certainly not top optical quality. You can get optical flats which are excellent. They come in various qualities from what is essentially selected float glass (moderately expensive) to tenth wave flatness and better (very expensive).