Don't get me wrong, Demon's Souls is amazing, both of the Soul games are among my favourite games this generation, and it is definitely worth playing if you haven't already. But that's the thing, it's been out long enough now that those who want to play it already would have.
The Japan version came out over 4 years ago, and the North America version came out just under 4 years ago. And for all of us outside of those regions, we got a release in 2010, although most people who wanted to play it had imported a copy by then. You could probably get a retail copy now very cheaply.
When PS+ made Batman available on the PS Store they skipped Batman Arkham Asylum and went straight to Arkham City (At least in the EU store). It made sense since it was the more relevant title.
For those who haven't played Demons Souls, you aer in for a real treat. But part of me is surprised that they went with this instead of Dark Souls.
Check out my investigation into 7th generation exclusives here: http://rustygamer.wordpress.com/