One thing about
4e is that high level encounters no longer usually equal quick tpks for the party like the days of old. While the party might not be able to win, usually the message gets sent across during the battle and they get the chance to run.
The fun for many people in the old "villain in a castle" scenario is the planning. How do we beat these heinous guy with what we got? In this case, the party has decided to try and recruit the royal guard. That's cool, now let them see how their plans have worked out.
Even if the party has a "boring" fight in that they don't contribute very much, they can feel cool and smart that their actions allows them to beat a guy they should have never beaten. For some groups that can be a whole lot of fun.
Or if the bad guy still wins, it drives home how strong he is, and will make defeating him one day all the more satisfying.