Nah, it's that it gives you a sense of progress and position within the game's "skill system."
I can understand it, but honestly I think a games replayability will depend much more on the gameplay itself. I always play games to have fun, and DmC allows me to have a lot of fun. I like Devil May Cry games because of the stuff I get to do, not how well I'm doing it. I'll improve because I want to, not because a system told me I could be better.
I'd say it's a little bit of both for me. I do like to improve my rankings in DMC4, but I'm not so anal retentive that I won't quit until each level of difficulty is shining with S's. Some gamers do it without so much as breaking a sweat. However, I have set specific parameters for myself.
-Beating all of the Secret Missions (it took me awhile, but I did finally accomplish it, and it did make me feel good).
-Getting all S's in Devil Hunter. I was beyond ecstatic when I did this, because Mission 20 (the final battle against Sanctus) gave me so much grief not just in the sense of rankings, but in general. He's such a cheap bas*ard.
-Beating Bloody Palace (My hands hurt so bad after that; I am NEVER going to go after an S, there. It was bullsh*t enough just clearing all 101 stages! I got a D, but it doesn't bother me.)
Goals I'd like to achieve, but I'm not likely to keep pushing at them:
-All S ranks in SOS and DMD mode.
Realistic goal? Bring all lower rankings up to at least an A in both modes.
That being said about the ranking system? I spend more time just d*cking around for fun, laughing while I take out lesser enemies while practicing with some of my weaker combo sets.
The ranking system isn't all important, and it doesn't make or break the value of a gamer; for me, it's all about personal goals.