Stack Overflow is *driven* by incoming links—ranking SO results high in Google is crucial—and it is those incoming links that bring people to the site.

We really should take your comment about *"not getting the Google juice they deserve"* to heart.  Stack Overflow is about making the Internet a better place and giving back. 

`rel="nofollow"` serves a purpose but, perhaps, there is some criteria that can determine when a post surpasses the potential for wide-spread abuse and can be placed in the "giving back" column. Some combination of:

 - Time
 - Voting
 - Reputation (i.e. trust)
 - User History (i.e. tracking tendency for a user to link to the same domain)
 
I'm not pretending to *know* the criteria, or even if there *is* a criteria. **But agree with the *principle* of your request, though.**