First, there are countermeasures. As a user on Google+ I'd quickly remove someone from my circles if they over-shared low quality content. Furthermore, Google+ can implement algorithms that affect the prominence of the content based on various quality factors. There's evidence that Facebook has such algorithms in place (1).
Second, the headwinds to gain share in a market with strong network effects outweighs the risk of a flood of content. Users already have a preference to use Facebook and Twitter given their network value (2). Any friction in sharing via Google+ just further encourages users to invest the bulk of their time and resources on these other platforms.
(1) http://blog.hubspot.com/blog/tabid/6307/bid/32124/Facebook-C...
(2) http://venturebeat.com/2012/02/28/google-plus-ghost-town/