Is this simply a scheme by Microsoft to inflate market share numbers?
Yeah, that was actually my first thought too, but the thought is flawed.
You are suggesting that MS would misuse sales numbers of HTC's dual-boot Android/WP devices, to report artificially inflated OS share. The thing is, OEM sales numbers aren't really used to measure OS market share. OEM sales numbers are used to judge OEM market share and OEM success, which is certainly also important, but those aren't the numbers MS would want to inflate.
The numbers that do matter, in regard to OS market share, are those published by Kantar, ComScore, etc. Their numbers are ascertained by monitoring web traffic, and are thus influenced by the OS people use to surf the web. Their measurements would be completely unaffected by such dual-boot devices. If a person never boots WP8 on their HTC One, they will never be counted as a WP8 user. These analysts also tend to publish numbers that are rather similar, so I just don't see how MS could get away reporting numbers that are 5% higher than everyone else's. It's just too obvious.
Finally, if MS really wanted to artificially inflate the WP sales numbers in that fashion, then HTC definitely isn't the right partner. If that was truly their intent, MS would have taken that offer to Samsung, and as far as we know, that didn't happen.
No. The idea is a different one. Just keeping more phones in the market and on shelves is probably the biggest reason. Letting people use WP8 without any risk is probably another reason. Likely there are more.
I would instantly buy such a phone, not because I would want to use an Android device, but because it would be a great way of comparing WP to Android on identical hardware.