I've edited my first post, gathering most aspects of the issue.
The reasons why Valor for kills should be given in every world PVP zone, on every planet are the following:
1. Preventing the overloading of specific planets/locations.
2. Adding more diversity/interest to the game. Especially on 50 level.
3. Assuming that Valor gains would be scaled accordingly based on the levels of the participants and the party size, this will result in lesser "ganking" of low levels. The higher levels would have more reason to fight their own levels or at least levels close to their own, as it will be rewarded in Valor points. In other words - fair fights would be more profitable.
4. For those who think that this will result in unfair farming of valor — the warzones would still bring more Valor, even with lost battles.
That's what I could think of right now. Also, I want to add that in all games (MMO or non-MMO) you get something when you kill an enemy. And when you kill and gain nothing you start to think you are playing some beta-version.