Here are some of the reasons given for bringing a rogue along:
Anushka of Kel'Thuzad: There's a lot of nasty things that can be interrupted in most heroics. Though I wouldn't take more than one rogue.
Khadros of Frostwolf: To listen to the sound of theirs daggers going schlick schlick schlick.
Morgrimm of Korgath: Because one of my friends is a rogue.
Owari of Frostwolf: To DPS, of course.
Mypetgoat of Bladefist: It's one expendible DPS that won't roll on my gear.
Madia of Maelstrom: They need loot and stuff too.