Well hold on now, I never said there aren't any reasons to stay with a subsidized phone. QuietlyBrilliant was going through the thread posting about how silly it was for anyone to buy this phone when it 'made so much more sense' to just get a subsidized version of it from your carrier. I was simply pointing out why I, and I'm assuming others, would do something so silly as paying the full $650 for a phone off contract.
That being said - even your personal example is a little tenuous. Even given the fact you have a special situation with your discount you still need to realize that you're paying an extra $30 per month for that unlimited data.
Prepaid (on ATT) unlimited text/calls/data* (data is actually throttled around 2GB):
$45 a month
Your Plan:
$75 a month
You pay $200 for the phone, I pay $650. So far I've paid $450 more than you.
2 years of service for me (the length of your contract): $1080
2 years of service for you (the length of your contract): $1800
Amount I'm saving over you in plan costs: $720
Total I've saved over you: $270 over two years. Now imagine how much the savings would be for someone who didn't have a discount.
Is unlimited data worth an extra $270 to you? Sure, it probably is. Personally, I'm coming from Sprint and the network is so bad it's pretty tough to even use 2GB of data if you wanted to... so it's going to be plenty of data for me. I'd rather pocket the $270. On top of that, going the prepaid route has the advantage of NOT being in a contract which comes with all of the advantages I wrote about in my last post. Since I'm going to Spain for a month at the end of the summer, the reality is I'm going to save even more money by not having to pay for phone service in the states for that month.
Sometimes it makes sense to go subsidized - especially if you are in a family plan. For others it makes a lot more sense to do something like this: best hardware, best software, cheapest plans.