Another use-case we've found is for customers like insurance agents where their clients need to send in pictures from a phone. They can easily send them by MMS to my customer's main number and then the customer receives it in their email.
Also, with the version that is on Github, set correctly, any softphone that uses SIP SIMPLE can be used for both internal messaging between the customer's agents and also for external SMS. It does have the drawbacks mentioned above in that it doesn't play well with group messaging and syncing between multiple users is a problem. And it doesn't currently have a web interface to send/receive. But for most of our use-cases, it is "good enough".
I'm not the original developer, but I have added a few things to it, and I can see where the web interface could be added to it, if someone had the time. (I don't, unfortunately.) And there are a few other niceties that I wish it had, such as a threaded view of messages and full-on support of MMS attachments to the client, instead of just to email. Again, as-is, it is "good enough" for what we need.
Regarding the Messages app that Mark has added: We haven't had a chance to try it yet, but I believe it does have the web interface already. And while I do understand the reluctance to put out $300/month, here's how we look at it: About once every 12-18 months, we typically need several hours of help. The last couple of times, it was 6-9 hours. We could have paid several thousand dollars to get that help at the time, but instead, Mark lets us pay it out $300/month. And then we also get access to the other benefits. Also, if you are going to pay $250/month for Textable, maybe it's worth the extra $50/month to get the built-in Messaging app and support the project in the process?
Don't get me wrong, I'm not defending Mark across the board. I don't agree with some of his decisions. But if this is the main software that runs your business, $300/month is probably not unreasonable to make sure it keeps running and stays secure. And while I would much prefer the idea of buying support on an as-needed basis, he does make a good point that in order to maintain a staff of folks to provide that support, they need to have a more predictable cash flow. Since most phone service is sold on a recurring, monthly basis, I can certainly relate with that.