I like wit.ai but I don't think their pricing model is sustainable for many developers.

At bulk pricing you get 25000 queries/day for $1500/month. At 30 days/month it comes down to $.002/query.

Assuming you generate revenue from advertising and you are able to display one ad per every query. You have to do $2 CPM or better to break even. Which is about the current market price [1].

There are other revenue models than mobile ad, but taking that example the developers would have to give all their revenue to wit.ai which would not make any business sense.

[1] http://www.quora.com/Mobile-Advertising/What-are-current-mob...