Why doesn't Duolingo reward you for translating 10 web pages say by allowing you to translate one web page in the other direction?
Such translations would help you learn to make up your own sentences, even if they are not helpful to Duolingo.
As Duolingo would probably want to ignore such translations, it could provide autocompletion to help learners without worrying about skewing the results. See for example: http://googlesystem.blogspot.ca/2010/09/google-scribe.html
This autocompletion feature could take into account general completions as well as those specific to the web page being translated based on high quality translations.
As for forming bad habits, you could always look at the high quality translations to compare them with your own.
The better the site is for learning, the more translations there will be. So in that sense, this will be helpful to Duolingo.