There are many pros when it comes to having a workout partner to help you spot on a given exercise. Probably the biggest advantage is that you can train to the point of failure and even beyond that point. Another advantage of having a training partner must be the motivation.
There are other reasons as well which will depend on what kind of person you are and how disciplined you are about getting to gym. When you have someone who is meeting you at the gym so that you two can train together it certainly will motivate you to get there and not let your partner down.
The next advantage or pro of having a training partner train with you is that he can advise you are the correctness of your form and that if you have a tendency to cheat when lifting heavy or use momentum then a training partner could be an invaluable support.
The disadvantages or the cons of having a training partner train with you is that you become dependent on that person to complete your workouts correctly. But it does not mean that you have to not train just because the person that helps you train cannot make it.
The problem is that you are more likely to feel discouraged and miss a workout if you're partner can't make it. The disadvantage of having a training partner is also that you cannot be completely spontaneous and go to gym when you feel like it as you have already made an appointment with your training partner.
You cannot, or rather should not listen to a Walkman or an iPod when training with a partner. They will probably be talking to you and you need to always listen to the motivation and the ideas of how the training is going between the two of you.