Is it really so hard to empathize with people who have different skill sets than yourself? Yes, C would be more efficient. Yes, this wouldn't be very cost effective used in a high volume product. Yes, it's a little funky to write a web page just to plonk your script tag in there.
But, if you spend all day doing front-end dev with jQuery and just want to prototype some blinky lights or a flood sensor in your basement, now you've already got all the skills you need. You're not producing a million units, you're not worried about power so much as it'll be plugged in. An evening with a soldering iron and your text editor and off you go.
Anyways. For prior art on this ecosystem, see janOS:
which is basically stripped down firefoxOS which runs your app only (none of the launcher crud and such) in god-mode. It's actually quite fun to go grab a $30 phone off of ebay, and have a nice programmable screen and sensor suite with wifi, gps, and cell connectivity. Indeed, I find it more compelling than this board they're releasing, because I like taking advantage of the cheapness of mass manufacturing. The only downside is it's difficult to impossible to break out any GPIO on phones.