As a guy that has played both the nice guy with one girl, and the bad boy with multiple women, I think it's time to talk about why being a player has far more benefits than being a guy with just one girl:
If you're a completely single guy, or a guy with just one girl, you're going to be needy. It's just a fact. There's no way to eliminate it. You're going to eliminate your only source of pussy if you lose the one girl you have. And being single, you're going to grow more and more desperate, which is going to show in your body language, attitude, everything.
With multiple women, you eliminate neediness. You're free to ignore girls, not chase them around, and not cater to their every whim. And if you lose one of them, then so what? You have other women.
Not only do you have sexual variety, but you save yourself from being a sex hostage. You guys in serious relationships know exactly what I'm talking about. The first thing a girl will try to do in order to assert her power over you, is take sex away. If you have multiple women, her taking sex away is a non-factor.
When you get into trouble, we all know that it can be difficult to rely on your boys. And if that one girl happens to be really upset with you? You're fucked. Women will do anything for a guy they love, so it's nice to have multiple insurance policies (i.e. multiple women) if you ever run into trouble.
The most difficult time for me to pick up women? When I am completely single. However, when I have my harem built up? Piece of fucking cake. It's the snowball effect. Those who have, attract more. And those who have little, eventually lose it all.
Which would you rather do, chase women, or have women chase you? You can either waste time and energy on them, or they can use their time and energy chasing and trying to please you. Think about it.
The only time you shouldn't be a dating player is if you have children with the current girl. At which point, your priorities change. Are these Machiavellian tactics? Yes. But they're no different than some of the "justifiable" tactics women use to control men. Play or be played.