actually I am thinking how most of Europe works. In Europe a lot of people buy unlocked phones because they are available and easily accessible. So to assume that only a small percentage would opt for this isn't necessarily true since other markets do support that aspect. Also if only a small percentage of people would go this route why do Verizon, sprint, T-Mobile, and att fight tooth and nail to maintain carrier locked phones, locked bootloaders, and locked everything. The carriers know that unlocked phones mean that consumers would actually easily change ship to better plans. Right now they have a monopoly by only offering locked phones which they will do anything to protect, suggesting that the threat of unlocked phones is much higher than a small percentage.
You are right I am thinking for a consumer stand point but I am also thinking about how the general consumers are going to buy phones. Who would by a buggy phone, disappearing keyboard, sms exploit, with potentially no hope of fix ie att not offering the 8107 update. So by kissing att corporate behind Microsoft is allowing att to jack up their platform. So now Microsoft still can't get a fair shake. And if you offer unlocked phones you can attract those customers who want the platform even though their carriers don't carry it. With the high probability of att screwing up the platform by blocking updates and loading up bloatware soon no carrier will want windows phone. Or already ending phones life, focus s and titan, about one year after they are released. But who knows maybe the 900 shall be such an amazing phone that no matter what it shines through.
but Microsoft does offer the 800 unlocked with the entertainment bundle. So they should at least offer that to students for a discount since no carrier wants to carry it even if they don't offer any of the other phones.