I just got Dura-Ace 9 speed bar end shifters (SL-BS77) to go with a Tiagra rear derailler and an Ultegra 9spd cassette.

I was under the impression that everything should be fine with that set up, but the very first instruction with the shifters is "This shifting lever cannot be used with any rear derailleur other than the Dura-Ace."

Does Shimano just want me to spend more money or is there any good reason for this?

Thanks!