There are two types of telescopes: high power, and low power. The high power telescope is used for observing planets, lunar details, and double star systems. Photography, when done, is by eyepiece projection. The primary mirror is shallow. Because short focal length eyepieces are used, the actual image area is very small-- typically 1/2" (12mm) or less. In this kind of telscope there is frequently a requirement to discerne dark features on a lighter background; any scattered light would reduce the contrast and defeat this goal. Since diffraction is a source of scattered light, you want to minimize it. This is done by minimizing all obstructions in the light path, including the central obstruction of the diagona and the gigantic scratches that the spider vanes are equivalent to. So you want a minimal diagonal and a guitar-string spider.
However, the low power telescope is a different kind of animal. The primary mirror will have a deeper curve and a shorter focal length. There may be a field corrector lens to sharpen the extended image. Normally you will be observing star clusters, galaxies, and nebula. If you do photography, it will be at prime focus. YOur image area will be as large as possible, the diameter of the focuser-- this is usually 1 1/4" or 2". At such low powers, diffraction does not matter. In photography, diffraction is dominated by the size of the film grain, typically 0.001". In visual use, diffraction is dominated at the center of the field by the resolution of your eye, typically 1 arc minute. In other parts of the image, diffraction is dominated by coma. So there is no need to worry about diffraction. In the low power telescope it is desireable that there be no vignetting; that is, a star should look equally bright no matter its position in the field of view. This is because variable star oberserving (one of the few activities where professional astronomers rely on amateur observations) is a productive activity that makes a significant scientific contribution. In variable star observing, the brightness of a star is estimated by comparison with that of other stars in the same field of view. If those stars are at the edge of the field and are dimmer, your observed brightness will be too high. If the comparison star is at the center of the field of view and appears brighter, your observed brightness of the subject star will be too high. Variable star observers work to 0.1 stellar magnitude, or about a 9% light difference. You cannot tolerate much more than half of this amount of vignetting at the edge of the field. That is why I advocate ZERO vignetting by the diagonal mirror for low power telescopes.
You can easily calculate the appropriate diagonal size for your telescope by clicking HERE for an Excel spreasheet. If you have a Palm or similar handheld computer, you can click HERE for a version that runs under hotpaw basic.