Interesting. If I eventually get a upgrade, not sure I'll ever turn on the cellular feature. Not going to pay $10, same as my iPad, for a watch. I'm more of a $5/month person for something that will rarely use cell data.
According to AT&T's site:
"Limits: Purchase limits (as low as 2 installment agreements), eligibility, credit approval & other restr's apply (see your Retail Installment Agmt)."
At one point last year I had 3 items on the next plan, iPad Mini 4, iPad Pro 9.7 and iPhone 7. The iPad Mini had like 2 or 3 payments left on it when i got the iPhone 7, but prior to that I did have the iPhone 6 on it as well.