Even though the UNSC does have more vehicles compared to the covenant faction and promethean by a landslide, that doesn’t necessarily mean they couldn’t add in the UNSC Hornet, SparrowHawk, and even the broadsword as a way for Spartans who rather use their military’s own grade A tech. Either way the hornet, if brought into Halo 5 could help as a minor powerful air vehicle and could play a similar role to the phaeton being able to clear infantry, but not having powerful armor.