hmmm, I disagree. It depends. I find that the only thing deathmatch improves is sniping skills. Honestly. It allows you to become an offensive sniper and those are really really good. Other than that, i really don't see it helping too much.
This is because: 1. You rush with low hp, don't care about dying. You spawn again anyway
2. Learn much slower when it comes to game sense ( where opponent is, how many, footsteps etc.)
3. Your style of play will change. we adapt and change to suit the style. E.g you wouldn't walk a lot in deathmatch so when you play another type of the game it will take time to pick up again
4. Tactics. You don't learn very much do you =/. These are essential in other forms of the game and scrims etc.
Overall i really believe normal matches improve you more.