Yeah. Because they were incredibly important events for their respective franchises from the minute they happened. Events that are so big that there's a before and there's an after said event. That's why they've been adapted multiple times. Other franchises have them (including the Flash franchise), even if not that well known by the public yet. Think Hal Jordan becoming Parallax; Green Goblin killing Gwen Stacey; and, speaking of Flash, Barry Allen's death during Crisis, etc.
Yeah, but I wouldn't say Glider's really done anything of note since her return. And Magenta hasn't really returned. She had one issue with Wally dealing with his out-of-place status in current continuity and then didn't make another appearance. Either way, though, she's Wally's villain and as long as Barry is the main Flash, I don't see her being important as a villain.I would say they're nowhere near as well-known as the Sirens. Silver Banshee (who hasn't been seen in years), Livewire (who is hardly ever used these days), and Granny Goodness are probably the most prominent in that list.
Glider's fridging was undone by the New 52 so I don't think it matters anymore, especially when she's been a pretty prominent member of the Rogues as far as current continuity. She's on the team more consistently then I think Captain Boomerang has by this point.
Magenta came back so I don't think it's that big a deal.