She was created as Spoiler, but killed off as Robin. If she'd been brought back like one or two issues later, I'd say whatever for that, but wasn't she dead for like years? A retcon doesn't erase that kind of thing in the minds of fans.
I would also say she's significant to the role/mantle of Robin, because I think her being killed off while in that role was a good bit of what did not sit right with folks. Probably the most flagrant woman-in-refrigerator moment in comics history, because Jason's death was cast as this super-rare and tragic instance in the history of the Bat-family, and it should -- because how many kids would need to get murdered under this guy's watch, before he starts to look a bit less awesome? -- while Stephanie's death was more kind of a "my bad" kind of thing, as far as how important it was to the overall Bat-mythos.
I think that makes her a pretty important Robin though, because it was because she had been a Robin, however briefly, that made people say that shoving her in a fridge was not okay.