How many times have you received a message basically saying this:
"You didn't give me an S, why?" and go ranting on about it
Its okay to ask why did you give me a D or a C, but not to ask for an S
Unless someone is being a complete jerk (such as intentionally summoning me into a Black Phantom trap), I give the player an S. To me, the rating system isn't about skill, it's about co-operative effort. Even when partners die, if they play with honor, they'll get an S from me.
Unfortunately, there are a lot of lazy players who don't take the time to select a rating, they just mash buttons to continue after a boss fight. This gives the other player(s) a lower default rating. On many occasions as a Blue Phantom, I've done a great job and not received a deserved S rating. In the grand scheme of life, or even in Demon's Souls, it doesn't matter that much, but I can understand why players feel insulted by not receiving an S rating for an S performance. I don't care about the amount of souls I receive, but I do care how my soul sign looks when a Host is considering whether to summon me.