A power meter mainly quantifies how badly you suck. As you train harder and better, it will indicate the degree to which you start to suck less. Seeing that you're starting to suck less can be a powerful motivator. There are other way of doing this, like timing yourself up a particular hill or along a stretch of road. You can also use the power meter to monitor your training to make sure you're putting in the optimal amount of effort for the particular exercise/intervals you're doing. This is way better than using heart rate, though heart rate is way better than nothing.

It's also a good tool to keep track of calories burned if you are counting calories to manage your weight.

So if you plan on using it as a tool to maximize the benefit of your training and quantifying the fruits of your labors, they can be a good thing.