I think the important thing to remember is that everyone is not same. While you may not find enough of WoW's content sufficient to keep you entertained permanently, others will.
Personally I have been subscribed, non-stop, since I started playing the game. I have never felt like there isn't enough for me to do. If anything, I often just find myself overwhelmed with too much stuff to do. So I actually quite enjoy the lulls between raid tiers because it gives me time to go back and catch up on some of the stuff that I didn't get around to earlier.
And if you're probably not going to unsubscribe anyways - then buying a 6 month sub makes perfect sense because it saves you around a month's subscription anyway.