2nd I travel alot - so I'm often not able to access a data connection, or I'm on a plane, or I'm on the London Underground. This is probably not uncommon for people that are learning languages. Even when on a train 3G connection is pretty inconsistent. Commuting is when I'm most likely to use Readlang. I'd say the captive audience on London Underground everyday is worth it alone.
I've already discussed this offline with OP, so I know that part of the issue is that you can never get the sentence translating power of Google Translate offline, but single word translations (plus the flashcards) might be possible.
You only have to look at the Duolingo community to see how eagerly anticipated the offline ability was. Same for Memrise.