It's MicroSoft, M$, ... whatever you want to call it.
1. All their repair work is almost certainly sub-contracted.
2. It's cheaper to hand you a refurbed Xenon, than a brand-new board. (blame the accountants)
3. There's probably fine print somewhere stating: "When you send in your 360 for repair, the replacement will be a model motherboard that is no older than yours..."
So basically when they get your machine, they verify which version it is --and put it in the enormous pile of units of that version to be checked. Then they got to the smaller pile of repaired machines --same version as yours-- and pick out one to mail to you. If your original machine is repairable, they'll eventually give it to someone else who sends in one.
This is pretty much how any business would do it. They want to eat as little cost as possible. (Although it would have been great marketing, if they'd given you the option to move to Jasper or Falcon --for a set upgrade fee.)
The gripes with MS, IMHO, are:
1. They had the design flaw(s) to begin with, and don't seem to have been in any hurry to remove the flaws.
2. They used so many cheap parts. (They've also been very quiet when/if they decide to discontinue using a specific parts vendor for a given part.)
3. They took so long to admit the problems were common and expand the warranty coverage.
4. The rates they charge for repairs not under warranty.
5. The ridiculous cost of accessories. Especially the proprietary garbage, that only lets you use MS branded storage.
6. Keying the optical drive to the motherboard [which I guess is to prevent copying], when the drive is the easiest component to break & replace. The drives are also less quality than what you'd get in a store for the same price.
I understand your disappointment, but it doesn't seem like you've got a genuine new or different complaint to add to those 6. You would have been lucky, to get a Falcon in exchange...
I don't blame you for thinking PS3. Although again, IMHO, blue-ray hasn't shown much ---you're paying Sony now for technology the PS3 may not fully use for another 3-5 years, so you're paying at least 4x what the cost of the drive will be, when software catches up to it.