Chas_m, I think in this instance why nordlp wants is not unreasonable. I, too, have dual monitors and it was becoming a real pain to have to move from the external monitor where the app was running, to the laptop monitor where the menu bar was located. Yes, it's only a mouse move, or a trackpad move, but it was distracting when I was in the middle of some complex Photoshop edit and wanted to save, or change tools for something I was doing. It's not that I want OSX to act like Windows (shudder) it was just that it seemed a omission in OSX that the menu bar was only on one monitor. Yes, I can go to System Preferences and move it to the external monitor, but then the dock moves, too, and I still have the problem of having to move from the laptop screen to the external monitor if the app I am using is on the laptop. Given how well OSX supports multiple displays, it would be mighty handy to have the option to have a menu bar on each.
For nordlp: SecondBar works well and has only one side effect I have seen. When SB is running and if I have something with a column display up (say iTunes with tracks listed in columns) and I right click on the column header row, the app expands to fill the work area. Not full screen, just full work area. It can be resized, but ti is distracting. I pretty quickly learned NOT to do that, but it's the only side effect I see in Mountain Lion 10.8.3. Also, if you have any apps that add an icon or information to the menu bar (DropBox, for example) it only shows on the main menu bar and not SecondBar. But that's the design of SB, not a bug.
EDIT: I agree that there is no reason to run two instances of any app. That is, as chase_m said, wasteful and unneeded.