I'm totally intrigued by the TextBlade too, but I can't help and think if I'm going to carry it around in my pocket so I can type on it anywhere, I'd want to be able to type with it even when there isn't a flat surface to put it on. Imagine if you are in your car and an email or text arrived and you want to reply immediately, or you're walking down the street, or in a park, or anywhere that doesn't have a flat surface for you to place the TextBlade on.
For shorts typing sessions (answering a text message or email), you have the on-screen keyboard.
For longer typing sessions, it is assumed you will find a suitable surface. I think it's a reasonable assumption. If you are going to spend 30 minutes typing, investing 2 minutes finding a place with a flat surface is not too much overhead.
I'm pretty sure people will get inventive with this, for example using the tablet's carrying case as a surface to type on.
Naturally if they improve their design so the TextBlade can be rigid, that will be even better.
They have already nailed a number of good ideas in the current design, and it looks like future versions will be even more interesting.
But they have to start somewhere, and this first version already has a lot for it. On paper at least...