Why do we need more 5 mans? Because half of this expansion is for characters under level 60. Yes you can go back to all the updated zones and redo the quests at 85 but to me theres a payoff missing for doing it. If you're leveling you get money, rep, xp for experiencing the newness. At 85 the money is usually not even enough to buy a stack of reagents, the xp gain does nothing at max level, and rep while nice if you're not exalted with a faction...may not be enough for some people. Blizzard has added so many things to help us level fast that we tend to skip complete zones while leveling and miss some of the new and interesting story lines.
With Heirlooms, dungeon quests and Guild XP gains, you barely get halfway through a zone before you are too high to really enjoy it. Yes you can always not wear heirlooms or run dungeons but that still doesn't address the fact that once you hit 60 you are no longer enjoying leveling because you're stuck in Outland with it's lame quests until you hit 68. And then you get to experience Northrend all over again except this time you can't abandon it at 78 for the new zones. You're stuck dinging 80 the old way. It's like the developers want to remind you that they have gotten much better at the game but they do this by making you go through old content.
Once you hit 85, most people are probably not going to go back through zones to quest and see the story lines. They will probably be too busy farming heroics for gear and rep which will lead them to over heroic themselves and burn out. There isnt a whole lot to do at 85 other then heroics and if you are really grinding them out to get geared you get bored of them real fast. We need new heroic instances to keep us entertained.
Update: So while I was busy with work and unable to post this yesterday, Blizzard decided to read my mind and release information about the new heroic 5 mans that they are adding. And by releasing information I mean it was all data mined by people who do things of that nature. So this can only lead me to believe that Blizzard is a fan of my work and has planted spy software on my computer to read what I write so they can make me happy. =D