Problem with the vehicle design is that they have to submit to the power of game balance. 
  
Just about every single vehicle that has been in the main games makes little to no sense in its design. 
  
Warthog - isn't covered, and the turret guy is completely exposed 
  
Mongoose - pretty useless overall all things considered 
  
Ghost - again not covered from top or sides 
Hornet - slow and exposed, uses a form of propellers 
  
Falcon - Uses propellers and is again very exposed 
  
Banshee - Incredibly slow with poorly designed tail 
  
Wraith - Mobile Artillery acting as a tank... and its source of power is openly exposed on the back 
  
Chopper - same as ghost 
  
Spectre - same as Warthog 
  
Prowler - same as warthog 
  
Mantis - little armor and its internals are exposed like joints and wires 
  
Four in game vehicles maybe make sense and that would be the Scorpion, Elephant, Mammoth, and the Scarab. The vehicles in Halo Wars don't have to fall victim to this balance issue as players aren't necessarily dominating the battlefield with them. 
  
Battlefield can get away with this because of how it is designed. People can spawn completely armed and ready to destroy 3 tanks or a helicopter.