Thieves will offload the phone to someone using software explicitly designed to wipe electronics to be resold.
Whether they are wiping an iphone that happens to have touch ID or not is only relevent towards the resale price once it's wiped.
Clearly Apple marketing works, as it's somehow convinced a member of (I'd hope) a more technical audience that their electronics are somehow safer against thieves.
The point is that with TouchID (as opposed to no passcode) the thief will not be able to send porn to my mom or read my text messages before they wipe the phone.
Basically, a stolen iPhone is only worth the sum of its parts so they can be used to repair other phones.
So if it works as advertised, stolen iPhones and iPads will only be worth the sum of their parts.
did I just predict iOS8?
Apple are perfectly happy with the second hand market for iPhones.
I've just ordered a 5S. It's costing me £709. My iPhone 4S 64Gb is worth about £200 second hand. Even a new 8Gb 4S, the cheapest model available new, is £349.
Anyone interested in my second hand phone was almost certainly never going to spring for a new iPhone.
The market for second hand iPhones does next to nothing to cannibalise the market for new iPhones (which Apple cares about) and strengthens the iOS ecosystem (both by bringing in new customers who might buy apps, music and movies but also keeping customers away from competing platforms).
There's more upside than downside for Apple in second hand iPhones.