Yes, he does have a very valid point. There is also the fact that this type of unlock WILL unlock "unsigned" applications and it automatically opens the doors for the pirates. You can't have one with out the other.
This could scare away many devs but, one thing people need to know is the percent who actually would unlock/jailbreak a phone is different than how many people buy phones. Not everyone will jailbreak, even in iOS it's under 5% and Android is higher but, most times it's to get a stable phone or to add OS features that the OEM did not include. Not everyone is going to pirate but, some people will but, I don't think it will be enough people to make a true impact on sales for an app (unless it's a really expensive app with no demo).
I look forward to unlocking features in WP, not pirating software, I do have a dev account now so if I need to "side load" and app I can but, I don't really use it that much. 3 row tiles, custom colors for tiles, tethering on Verizon, and other little hacks like backing up all app settings (including games that you can restore if you nuke your phone), did this with WP7....
So, there is a lot of advantages of doing this, not just stealing software...