That would be overpowered. Your entire team puts on flak jackets and suddenly half the 3 kill streaks are worth dirt. With Ghost it's just a recon thing, you can still easily see a person with Ghost on and fill him with holes. RC-XDs are a weapon, a person with a Flak Jacket is basically invincible to every RC-XD that comes his way, which completely kills the purpose of using the RC-XD in the first place.