I think ScottC may be right. There is a difference in the in the Terms and Conditions between the Family and Individual Plans/Tabs.

The T&C for Family Plan has the following language:
"Unlimited Rate Plan Promotion: Minimum two lines required. Includes 5GB of capped Smartphone Mobile Hotspot tethering that cannot be upgraded."

The T&C for the Individual Plan does NOT have that language. The T&Cs do mention tethering but only to note that there is no tethering while roaming internationally (OK, sub-optimal) and possibly implicating you if you stream a lot of music via tethering (not me, if I'm reading it right).

Three drawbacks:
1) I've got two phones with AT&T right now. If I upgrade each of them to an iPhone 6 and put them on this plan, the combined bill is quite a bit higher than upgrading them with AT&T. Granted the AT&T is only 10GB per month for the two phones combined compared to "unlimited" for each of the two lines. But, on the "simple" pricing of the device itself, I'm paying a bit more (e.g. $99 to start with T-Mo then $27/month for 24 months v. $25/month with AT&T for 24 months). AT&T for 10GB and the new phones (and the iPad discussed in #2 below) would run approx. $190/month plus tax for the first 24 months. T-Mo would be about $99x2 to start then $220 per month plus taxes for the first 24 months (then, $160 per month plus taxes) PLUS the plan for the tablet below.
2) I've got a family member with an iPad who's currently on my AT&T plan as an add-on device ($10 per month) and eating out of my current AT&T data plan. Adding them on to TMo pushes things up a bit - esp considering that they use next to nothing data-wise.
3) We're still in magic T-Mo terrible coverage land. So, we'd be going with their WiFi station. A de minimus cost. Right now, we're using the AT&T Femto/Fento (sp?).

The plus side for T-Mo is the "free" international roaming. But, that's about it. I do a fair bit of international roaming and have been paying for it with AT&T. But, I'm getting better at "planning" my expected data use and buying the appropriate plan and, more importantly, using WiFi rather than mobile data.