Copying has it's limits, but I think it's quite valuable. It helps you get a strong foundation of being able to see and then recreate what you are actually seeing, and then get immediate feedback on what you did wrong when you compare them. It's about technical control.
By no means should it be the only thing you do, that's a given I would think. But recreating another person's work is hard, and you can learn quite a bit from it. You realize "I looked at this and did this but that didn't look like the original at all! How did he... oh, maybe... yeah, that looks right! Cool, I didn't think of doing it that way!"
Imagine a fighting game. There are a few ways to approach it - you can do "fundamentals", by learning the move lists and the principles of positioning and the reach and angle of attack for each character. You can learn by "doodling", just playing around with your character against the computer and getting comfortable controlling him. But there's another way to learn, one that shouldn't be ignored - studying the technique of those who are good at the game. Looking at a match and going "Okay - why did he do this here? How did he do this here? Huh, I didn't know you could do that at all..."
Imitation is not going to get you there on it's own, but studious imitation, studying the experts and trying to do what they did, is still, in my mind, an incredibly useful tool. Understanding the fundamentals and building a strong foundation is important to getting the most out of this, of course, butjust like with doodling - if you are doing so without any aim, desire, or knowledge, you will not improve.