The last time I checked, expansions were for the people at end game with nothing to do. This was to bring them back/keep them playing your game. The lowbies at level ten and such have months of content to play. We don't!!!
I don't think your premise is valid. Given the story driven nature of LotRO, expansions can be legitimately seen as a device to move the story forwards towards the conclusion that Tolkien wrote. And, indeed, inclduing the Battle of the Hornburg (*and* developing a mechanic to handle large battles, as are coming up) moves the story forward.
The inclusion of low level characters in the system permits Turbine to provide a reason for new players to buy the expansion, instead of waiting those months before they "need" it. If you can't think of anything to do, I suggest that the problem can be seen in your mirror. If Turbine were to delay HD for a year, I wouldn't run out of things to do in game.