In some airplanes, the numbers can be quite critical, for example, the stall angle of the wing of a replica Spitfire. The guy who tried to take off at Watts Bridge without lifting the tail wheel can verify this now. So yes, in some cases it's important to know numbers. However, in most instances with well designed sport aircraft, the pilot can tell by feel when it is wrong as something will be quite different to his usual flying experience. For example, the nose might be pointing way too high, or the controls feel way too light. Mostly it is near the stall that we recognise when things are wrong, but there are probably times when the very adventurous go to places they weren't taught, like doing aerobatics in a drifter, that a pilot may come to grief by not knowing "the numbers".