I don't get this one bit... I am comparing the plans now with what I have currently and I'd pay $40 MORE a month with the MSV compared to the MS. This is using the AT&T Rate Plan Calculator...
http://myattrpc.com
EDIT: You only get the discount ($15per/phone) if you get the AT&T Next Plan with the phone. So if you then add in the monthly installment of $25 a month (if you choose the 18 month upgrade) then it equates to $40 a month per phone. This is the SAME price as if you were to buy the phone with a 2 year contact and pay $40 per phone.
This plan charges $15 for all phones + $100 for the 10GB of data + monthly installment (either $25 for 18months or $32.50 for 12month). If you do not upgrade every 2 years, they both would still cost the $15/month out of contract or the standard MSV Plan. The only difference is that you get to pay the $15/month with no monthly installment for 6 months as the 2 year contract is 24 months. Leading the cost to the 2 year contract to be $150 more expensive ($25 x 6 months).