On that page the fine print says this:
"Claim your code by July 7, 2013. Code must be redeemed on your Windows Phone in the Wallet application by September 29, 2013. Unused promotional gift code funds will also expire on September 29, 2013. See Terms & Conditions for details."
So, you have to claim it by July 7th, and redeem it by September 29th. We all agree on that. But then you say that unused funds will not expired, but the fine print implies that it does expire: "Unused promotional gift code funds will also expire on September 29, 2013". That is why most people in this thread assume they'll lose any remaining funds if not spent by the end of that day. However, if you log into your account on commerce.microsoft.com you will see that the funds are listed as having no expiration, which supports your claim, but contradicts the fine print on the page you linked. Thus, we are all confused.
I have no problem spending this money on apps: and I've already done that. Bought tons of apps and games. The problem is that my wife doesn't care at all about apps or games so she has a huge remaining balance that she could slowly use up but it would take a year (as new apps are being released). There is no point in her buying a bunch of apps she doesn't need or like, so it would make sense for her to hold onto the money and spend it at her own pace. On the other hand, if the money really does expire on the 29th of this month, then she'd be better off spending it (which would also benefit developers, rather than Microsoft just reclaiming the money).