Yes, that is exactly it. He was more of a plot device. Unlike other characters he wasn't central, and didn't have staying power even through the one game. Julius however was central to the first game (as noted, I've never played, but know enough of the story to make some inferences), yet lived to near the end. He also left a mark on the Boss, because of everything.
You know, by that virtue, we need Oleg back. He's come across very early in SR3, he stays with you the whole game, has his own personality, AND he's referenced repeatedly in SR4. He certainly left his mark, Kinzie can't get over him since he was probably the closest thing she ever had to an actual love interest since, well, ever. Plus he even shows up at the start of the game. By all that I suspect he's actually going to show up, in a DLC. It's too much set up, and they've already shown they're happy to use stuff that came up before.
So I guess then the question is what characters that are central to at least one of the games haven't shown up yet? There IS a Dane Vogel mode, buried away... meaning he might show up in a DLC. Who else will show in the future (and moreso, who else has their models already in the game like Dane or Oleg?)