If Yahoo! (stupidly) pulls the plug on the Overture tool, which is looking more and more likely, then this new tool could easily help to fill the hole that the Overture tool will leave. It’s a pretty smart move for WordTracker, who could stand to inherit a huge amount of traffic, judging by the widespread use the Overture tool has received over the years.
Update: Search Engine Roundtable has a statement from Yahoo! that the Keyword Selector Tool is indeed on its deathbed, but they are building a new one to take its place.