I've beaten Sands of Time and really don't want to play the Warrior Within, but I do want to play the Two Thrones. I don't want to miss any major plot points, though. So do I need to play through this, or can I just go on to the next title without missing much?
I'd recommend playing it if you want to be up to speed before beginning TT. However, if you really don't want to, there are plenty of good synopsizes out there that will get you up to speed. Wikipedia's isn't too bad and covers most of the points.
However, even with the darker elements and the over-abundant fighting, it still makes for a fun play-through.