Well you have to end the trilogy somehow. If you think about it, there isn't really going to be a single battle where the rebellions wipes out every last remnant of the Imperial forces.... so killing the Emperor, and destroying several of the Star Destroyers in the process is the next best thing. In one of Timothy Zhan's novels, he suggests that the Emperors will kept the troops organized, he used his mind to control thier movements; so once he was killed, the Imperial troops would have been in disarray, and those that weren't killed or captured would flee in panic...

How does that sound? :-)

Oh, and to answer your question DC: "Why do the Sith insist on giving everyone the name Darth?"

"Because it sounds cool..."