I can't wait for the day...
Even with Sprint's $20 add-a-line, the total contract cost is $480. If used to get a high end device, the savings are abound. I often recommend getting an extra line for a new phone to my customers, and they often buy. It's no different than leasing.
And then if they want to cancel, they have an exorbitant termination fee.
I do end up recommending adding the line to get the phone at commitment pricing sometimes, but I'm annoyed while I do it.
Also, if an iPhone 4 (or 4S), your SIM is not compatible with anything other than an iPhone
Customer has two lines with AT&T, both iPhones. Neither are eligible to upgrade for a while, yet they want the new iPhone 4S. I add the line to their account and add the data plan to that third line. This brings the price of the third line to either $25 or $35, depending on which data plan you pick, obviously. However, once I am done ringing out the third line, I do an IMEI and SIM swap on the third line to a dumbphone IMEI and a new SIM. I then swap the IMEI and SIM on the line that the customer wishes to use the iPhone on, and viola, they have a new iPhone for only $9.99 per month more.
We received 11.
Stupidity costs! Big!