Well, make sure to note that I said the drives I've had the most problems with are Western Digital and Hitachi. Hitachi, acknowledge they had a bad batch and I just got a couple of those.
Also, I ran a total of 14 drives in my two different system (one at the office, one at home). Both systems stayed powered on 24/7. I believe heat might of killed 1 or 2 of them, which is why I started running with the side off of my systems and put more fans around the drives. Though 2 of my hard drive failures were with Laptop drives
I've have some drives that have lasted 5 years (though I normally don't keep a drive for that long anyways). Plus the MTBF is more of a average not a Guarantee. I find the drives that fail the most (or should I say fastest) for me, tends to be the drives that I use the most. I've gotten pretty lucky so far in that all the drive failures that happened, happened while there was still a warranty on the drive, so while I had some data loss, I got a new drive at least
Don't know if it makes any difference, but I also tend to buy the OEM drives, though if the price difference is with $10, I'll grab the retail.
I should note that I've worked on some old PC in the past year that had 10 and 15B drives in them. Once system was from 99. So drives can last a while, espeically if you only power the PC on once every week or so