That's like saying Knowledge (Streetwise) has no use by itself. True, the use isn't as concrete as, say, Stealth, but it's always been up to the GM to determine how useful Knowledge skills are. It's entirely possible for Knowledge (Tactics) to be of use aside from the optional rules of using it to set Master Plans and set/detect ambushes. It could be used to identify someone's fighting style, for example, which can be used to identify the person and their instructors.
Master Plan isn't for "adequate tactical decisions", though, it's for truly innovative and inspired ones that give a decided advantage, the kinds that take a lot of intel and scenario imagining to work out. (And are usually made well before a battle begins, or is anticipated to begin.)