Google AdWords can be slow some times, going from page to page. The AdWords Editor is much faster in this case, since you're not waiting for anything to load.

Also, if you are updating a lot of bids, and you take too long, AdWords will log you out. You won't find out till after you hit the submit button and all your changes are lost. AdWords Editor prevents you from having to deal with this problem.

I noticed the AdWords Editor now will pull in stats on the keywords, which makes it even more useful.

It's not a complete replacement for logging into AdWords, but in some cases, it is much faster to use.