There are many philosophies when it comes to practicing guitar. Some people don't practice at all, others practice for 10+ hours per day! Steve Vai's legendary 10-hour guitar workout comes to mind. But consider this...who do you think is the better guitar player? The person who hardly practices, or the person who practices on a regular basis? Of course it's the person who practices regularly!
I'm a strong advocate of practicing on a regular basis because with good guitar technique, you can play anything! If you think about it, it makes perfect sense. If you can alternate pick most any any lick or picking pattern, then any time you’re trying to learn something new you’ll be able to pick it up much more quickly then if you don’t have good technique.
Obviously you want to practice things that you have difficulty playing, or create your own exercises that are similar to those things you struggle with. By focusing your practice time on these things, you will improve much quicker. Then in the future when you encounter things you previously used to struggle with, you’ll breeze right through it!
Think about it…if all you practice are power chords, you just really limit yourself as to what you can potentially play. By focusing on certain techniques, you’ll be able to do so much more. But you can still play power chords if you choose. I don’t know, maybe some people are happy only playing power chords, but I digress…
Now I'm not saying that you should go out and start practicing for 10+ hours per day. I never practiced for more then 3-4 hours per day. I think that what you practice is more important then how long you practice. Of course it’s ultimately up to you. This is just how I view the guitar.