I wouldn't really use Cain as a great source of fluff. There are a lot of contradictions in his books. For example, according to Cain and some of his soldiers, daemons are common knowledge when in the wider fluff entire planetary populations have been killed when daemons where on the same planet, regardless of the fact that those particular people didn't see any daemons.
However, since it's pretty much official GW policy that "there is no canon" in regards to 40k, it's just as valid a view of 40k as any other source - it just depends on what sort of setting you want to portray as GM.
Personally, I see the Cain fluff as being the more reasonable view of the Imperium, as the Guard is actually competent, the characters act like real people, and not everything is 100% grimdark, 100% of the time.
~Yea, Tho I Walk Through The Valley Of The Shadow Of Death, I Shall Fear No Evil~
Posts/views/opinions are in no way representative of FFG, and are entirely my own.