Actually, you're contributing to the uninformed thread. AT&T's new value plans aren't directly comparable to the old plans, and the $15 off bit is only valid when comparing value plan to value plan, not old mobile share to new value plan.
Let's do some math for a high data family plan like I use. 10GB shared across 4 iPhones is $120 + $30 per phone. $240/mo. Thanks to the ease of Apple in-store trade-ins, all four of these iPhones can be upgraded every two years for $0 out of pocket, but there's still AT&T's $36 activation fee. Let's roll that into the monthly cost: $246/mo gets 10GB shared data and four brand new iPhones every two years. Of course there was also an initial investment of $400-800 to buy four iPhones, but that's only one time ever.
On the new value plans, 10GB is only $100, and if you already own the phones, they're only $25 each. Monthly cost $200. Note that's only $10 cheaper per phone compared to the old plan. But cheaper nonetheless. However, the upfront cost was around $2400. If you elect to upgrade four phones every two years, your average monthly cost on the new value plans is about $290 (with 2 yr trade-in value factored in too).
The first two years on the old mobile share, with upfront costs rolled in, was $271, and then it's only $246/mo each year thereafter. Even if you upgrade those four phones only every four years, on the new value plans, that still averages to $256/mo. That's about the break-even point on total comparing old mobile share to new value plan, except one ensures your phone is never more than 2 years old while the other leaves you with a device that's lost all its trade-in value.
So, math indisputably proves that the new plans are more costly to the customer, at least the high data family users, in every way. And AT&T knows this. The move is one designed purely to increase their profits while placating customers with a slight of hand marketing show.