Mixed feelings.
It's NOT about ADA, although those served by ADA will benefit from the service rather than carrying a TTY device with them.
The movement for NG9-1-1 revolves around a number of things. First, remote notification of critical events. Automatic Crash Notifications (ACN) are part of this and are currently production in OnStar. Industry insiders also speculate that this can fuse with medicine and nanotechnology to "call 9-1-1" for EMS response when a heart goes into dysrhythmia, blood glucose levels go out of tolerance for consciousness (on either end of the scale), etc. But the biggest (and lamest) reason I hear for doing NG9-1-1 is to resonate with Gen-Y and successor generations. Back in the day (I'm dating myself here), the Telephone Pioneers came into first grade classrooms and taught kids how to dial "Operator." More contemporary programs taught them to dial 9-1-1 (and wasn't mommy surprised when the police car pulled into the driveway after Junior tried it at home). But we've failed to teach young people that their texting devices have this thing called a voice telephone, with a speaker to listen through and a microphone to speak into. So, we push technology (which isn't necessarily bad) without having an intact funding strategy. Given today's austere environment, and "no new taxes" with some jurisdictions having a declining tax base and some sunsetting their funding provisions, the burden of paying for technological nifties rests on the 9-1-1 ETSB (or local equivalent) instead of the proponent industry. In fact, the proponent industry runs away from its funding obligations, but doesn't fail to capitalize on marketing it's networks' ability to get you help when you need it.
It's also my opinion that, even though SMS may be geotegged somehow, that the "9-1-1 industry" will find a way to make it the tenuous house of cards that wireless Phase II has become, with this one doing database, that one doing connectivity, like a tree with twine strung between the branches connecting everything... only to provide a 1.2 kbps data spill after a CAMA number (policy limitations, not technological limitations). The motto of the industry? Everyone gets paid.
At the end of the day, it doesn't matter. It will happen because, absent education, it becomes the de facto standard of care within the community.