I've never really been a fan of levelling in FPS games. This is just my opinion and I realise people like them. It's like people think they're entitled to easier kills because they played more tho, when they should do already through more experience or simply because they have learned to play and aim better. Rather than feeling you're good because the game makes you stronger than people who haven't played as much as you.
Levelling is there in an operant conditioning fashion too. People get addicted to random rewards, and start doing the same things over and over even when they're bored of the game. I feel it muddies the reason people play games, and they forget why they're doing what they are in the game. Its more about rewards and KDR rather than helping the team to win.
I don't mind it if they are very small changes or side grades though. They didn't need to boost physical stats (faster ads, reduced suppression, reduced sway etc) and give unrealistic weapons and attachments though. Should have been slightly different looking stocks like more worn wood, or more scratches, painted sight etc. There are loads of suggestions for realistic unlocks that don't impact gameplay much, which would work in the same way as what we have now but without spoiling the feel or atmosphere.