I'm still trying to wrap my mind around the various $15, $40, etc. discounts/charges with this plan. But that's okay. It clearly won't save us money. My wife and I pay $115 a month for the two of us. I'm on unlimited, she is on 2GB, and we have so many rollover minutes stored up we could talk non-stop for the rest of the year!
The thing I see with the Mobile Share plan is that AT&T is purposely obscuring some of these discounts and up-charges when you upgrade. I know this is true, because many people switched to the plan and thought they were saving, then posted on the forums here shocked that their rate was going to go up $40 when they get the new iPhone!
This is exactly the crap Apple was trying to cut through when they did the initial plan with the iPhone. $50 + $30 for unlimited data. Simple, short, and sweet. You know what you're paying and there aren't any fluid "discounts" that might change in the future. I wish this had stuck a little better.
At least T-Mobile is now making strides to shake up all this mess and confusion. If their network were better, I would already have switched.
For now, I'm definitely keeping my unlimited. I don't go above 5gb, but the Mobile Share plan would have me paying about $50 extra a month.
So to answer the original question: No, the unlimited plans are not obsolete for all of us.