The thing with playing guitar is there is no one "correct" way of playing (aside of saftey factors where you might actually damage your muscles) If it's comfortable and works for you, then there's no really problem with it.
The only disadvantage your method has is if you wanted to pick a note higher as part of the riff. If your third finger was being used your pinky would be free to play the higher note......
That doesn't actually make a lot of sense, but hopefully you followed it.
I used to play the with that method until I realised it was more beneficial to my barre-chord to play the barre with my third finger, (for the reason I gave above, your pinky is "free" to play something else)
Now my dexterity is good enough that I can play power chords with several different fingers if I need to. I think the important thing is to not try and limit yourself to one method - if you can play it several different ways you'll be ready for almost any situation you might come across.
Hope that's helped, if you've any more questions feel free to ask me or anyone else (someone'll be able to explain it a damn sight better than me). Good luck and "rock on"!