It comes down to: What is the evaluation for? For a human using an engine to analyze, it is about getting to more win-likely positions. And for an engine, it really is the same; plus to guide the search. Having a perfect trinary win/draw/loss would certainly be the _truth_ about a position in some objective way, but it would almost certainly not be the optimal way to win chess games against a set opponent. 1. e4 and 1. h3 are almost certainly both draws with perfect play from both sides, but the former is much more likely to net a win, especially for a human using the engine.