Knights are basically the elite troops of (Mythic) Europe. This is represented quite well by their average stats (as given in Lords of Men, page 108) which eclipse their closest competition by around five points in both attack and defense, and who have the highest Leadership modifier when fighting in a trained group (giving them an even better lead). But, well, I'm not great at statistical analysis, and I don't know much about the perception of knights in medieval Europe other than that they're considered the elites. So how frightening a force are knights? Would a relatively small group of knights usually be enough to significantly turn the tide in a losing battle (say, five groups of knights reinforcing a group of twenty combat groups each of infantry, archers, and levies against a similar force twice that size)? Would the mere presence of a large group of knights cause a force lacking in them to have morale/discipline problems, any more so than they'd normally have fighting against a different group of trained enemies (say, infantry)?
Basically, how much of an impact do knights make on battles they're part of, and would the perception of them be one of fright/respect in most circumstances?