Imagine you're just a n00b and you're getting your shit kicked, but you catch a 211-47 player hopping thru a field and light him up.
Imagine the bad player retention increase when they get a huge xp kick for killing someone on a streak.
Also it'd encourage good players to seek out and fight other good players.
Seems like a win-win for everyone.
Like if someone is on a 4-5 kill streak, they're worth 2x the normal kill. If someone's on a 12 kill streak they're worth like 3000 xp. Or even just, if you're in the top 5-10 players in your lobby you give an XP boost. Damn, it could be so awesome. Planetside 2 did it and it was cool.