I've been on Cingular/ATT for almost a decade now and an iPhone owner since the iPhone 3g launch day. I currently have an iP4 on FW 4.1 with the 01.59 BB preserved. I'm not unlocked, but it's just a matter of installing UltraSn0w if I want to be at this point, and I like maintaining my BB purely for resale value.
I won't knock the OPs validity -- it seems like a perfectly legitimate way to go for some people in certain situations. My personal opinion is that it seems to involve a different, but roughly equal amount of hoops to jump through as someone who goes the subsidized JB-unlock route.
First of all, OP doesn't seem to consider that most people (disclaimer: I'll take the liberty of using the terms "most people" and "on average" to make my points as liberally as the OP has in their posts) simply don't have close to $1000 cash on hand to simply spend on a phone. $200/300 is much more likely, unless the purchaser wants to use a credit card, in which case you're looking at interest fees which the OP never considered. Personally, phone purchases come out of expendable income -- there's no way I'm dipping into savings for a phone. So, unless you're the type of person who has just short of a grand to spend on a phone, the OPs method won't work for you.
Second, the convenience of procuring a locked iPhone vs. getting a factory unlocked one from out of country may be well worth the $20 a month the OP claims someone would save. There's peace of mind involved in being handed the product you're purchasing at the point of sale. Wiring money to a friend who lives abroad to buy an unlocked phone from Apple and then ship it to you is extremely inconvenient for most people, and that's assuming you have a trustworthy friend abroad whose willing to do this for you, which most people probably don't.
Traveling internationally for the sole purpose of buying an unlocked phone automatically offsets whatever money you might save in owning an unlocked phone. So, unless it's the case that you happen to be abroad for some other reason and have $867 to spare, the OPs method probably won't work for you either.
There is the ebay/online route. But even if you're already in the "I have $867 to spare" club, you have to be in the "I'm comfortable spending that $867 in an online transaction with a private party re-seller who claims to have a factory unlocked iPhone 4, and I don't mind going through the fraud refund process with PayPal to get my money back in case I get screwed over" club. Most people probably don't fit that description either.
Put it this way. I jailbroke and unlocked my 16gb iPhone 3g I bought in July of 2008, then sold it to someone on T-Mobile for $250 (yes, I told them to make sure to check the dev-team blog before ever updating firmware). Then I went and bought a subsidized 16gb iP4 for $200. I'm on contract with ATT with no intention of leaving, and if I do travel abroad, all it took was a 30 minute investment of time using Pwnage tool to make sure I can unlock my phone whenever I want to. I guess iOS 4.2 has some cool features, but they're definitely not worth $467 to me, and quite honestly I don't think any iOS update really would be.
So yes, the OP's method should be fine for a very specific group of people, but since most people don't fit in that group, it's probably more convenient to go the subsidized/JB/unlock route.