For me this is a very good thing due to the technical aspects, not whether or not you can speak. A text message is just one single packet of data (140 bytes, which can encode 160 7 bit characters). At the lowest voice bitrate, a single second of audio is like sending over 50 text messages - PER SECOND. It takes a much better (as in consistent) connection to initiate and maintain a voice call compared to sending a text message. It also requires a lot more power. As someone who does a lot of hiking and dual sport motorcycle riding in the Appalachian mountains, I know first hand that often the only usable mode of cellular communication is SMS (and don't even dream about data with a bad connection - that's worse than voice even). Of course I also carry my amateur radio as a last ditch fallback if there was an emergency.
Another big advantage of SMS is your phone will keep retrying to send the message. With voice 911 you have to manually try over and over again until you can get a connection.
If you look at the death of CNet editor James Kim, and the miraculous survival of his wife and two very young children, you'll find they were saved because just a few packets of cellular data made it from their phone to a cell tower, and a diligent cell tech found that in a log file (which narrowed down the search and his wife and children were found as they were on foot trying to hike out). It's my opinion that they are alive today due to a rare and unpredictable phenomenon known as Tropospheric Ducting, which can temporarily reflect radio waves back down to earth (thus greater than line of sight) when there are layers of atmosphere at different temperatures in the exact right configuration (kind of like how you can see light shimmer over a road surface on a hot day - that is because of the temperature gradient of the air directly above the road compared to the air above it - same thing can happen with radio waves at a larger scale).
Anyway, there are many times that SMS messages can get through when a voice connection cannot. I will stop on my motorcycle and send a text, then maybe 30 minutes later it will finally go through as I temporarily get service on a mountain ridge, or my cell phone is simply rotated to a more optimum angle relative to the cell tower.