Costume and weapon variation is very important to me personally. Sure, theres two characters. But with a range of Devil Arms and costumes there is so much customisation its almost as good as seperate characters.
Think about it, in DMC3 you could play like this:
Regular Dante, with Rebellion and E&I
Coatless Dante, with Beowulf and Kalina Ann
LDK Dante, with Nevan and Artemis
And many, many other variations. With the styles as well, that meant that there were six ways to use every combination which drastically altered the play style. (I know Dante had styles in DMC4, but I'm comparing to the game as a whole, including Nero)
Although Nero is a great character, Dantes customisation in DMC3 (and DMC1 to an extent) gave so much variation that I could easily play through the game over and over while keeping it mixed up enough that it wasnt the same every time.