well if you need an actual reason for robin and nightwing showing up in arkham city, the most obvious thing is that Dick Grayson and Tim Drake heard about Bruce Wayne being incarcerated there and decided to go help their mentor and surrogate father in case he couldn't get his costume or gadgets. If your father were in trouble and you could help then wouldn't you?
Um, because he's Batman's side-kick some of the time. Because Robin can actually be cool, particularly when it's Tim Drake. Because it's not the Burt Ward 60s show version of Robin or the twerpy litte prepubescent kid version. Because you can also play as Catwoman and Nightwing, both of whom are awesome. Because the Bundles are side-missions: little sections that are woven into a game where you mostly play as Batman. Because the Catwoman, Nightwing and Robin Bundles are DLC so no one is forcing you to buy them. Jeeze. Save your nerd rage for something worthwhile...
If we are speaking of Tim Drake, who is the Robin used in this game, I actually like Robin MORE than Nightwing AND Batman. So... yeah... I'll be downloading this. Robin hopefully uses his staff and R stars. He out of the 4 Robins (No I don't count that brief stint where Spoiler was Robin) is clearly the best detective-wise... maybe his martial arts skills could use some work, but he's still young. He's at least as good as Batman was at that age.
Aren't Robin and Nightwing only for the challenge maps? Unlike Catwoman, who you can play as in the story mode? I'm debating spending points on the bundles because I don't play the challenges, just the story.