And all those Guardians killed by rogue Guardians, Crota, the Fallen, the Hive, the Vex, etc? Spartans are insane. Reaction times are just fractions of a second, they can process information in their brain incredibly fast, have a super-intelligent A.I. guiding them, run super fast, have reinforced skeletal frames supported by their armor, extremely durable, among other things. They are bred for combat, dogs of war. Don't get me wrong, I love my Guardians but Spartans can probably pick us apart with ease =\. Ghosts are extremely valuable partners but they can be separated from their Guardians and destroyed. To be fair, if Spartans and Guardians did square off, a few Spartans will fall owing to our Space Magic being unknown to them. But afterwards we're looking at warriors who just learned our strengths and how to counter them. We can only surprise them for so long before they come storming in.