Hi Steve,
1. Barlows have many uses and magnification ranges but I only use them to get the next eyepiece focal length in the range. I really do not know why manufacturers choose x2 as this will duplicate a given eyepiece(eg. 12mm to 6mm, 50mm to 25mm etc.) the best multiplication factor to give one the smoothest eyepiece focal length range is a factor of (twelfth root of 2) 1.23 something. The closest "Barlow" to this that I know about is the Tak x1.6 five element extender.
2. Re: "Max Magnification"
How high you take a magnification as being useful is a function of "Seeing", "Sky Transparency" and optical quality. The number max mag is 50(Aperture in inches) or 45(Aperture in inches) is very rubbery and doesn't say a great deal other than to tell us a magnification needed to see a split double star at the limit of the telescope's resolution. It tells us nothing about the contrast one would see when viewing the planets or the moon. High magnification, in good conditions, allows for scaling of the image so that one sees a "nice large" image.
Jerry.
