IMO, no.... I have never used it and get sites to rank in everything I need without the need for a tracker, but I cant speak for anyone.
I prefer Trellian to Word Tracker, but it still works well. I'd say if you are involved in regular keyword research/analysis then it's a worthwhile investment. I know some people quite like Overture Keyword Tool, but my advice would be to avoid this like the plague, unless you just want some relative figures for comparison.
If you are not sure about WordTracker, you can also use www.keywordspy.com for your keyword research tool - This will give you an opportunity to immediately track down your competitors and gather keywords for the promotional campaign of your online business. It offers Free trials. KeywordSpy is keyword research technology where you can also earn and even surpass what your competitors are making with Google Adwords and Overture. KeywordSpy gives you the key to their success: a good ad-campaign with the right keywords. It also has a ClickBank Affiliates Search Engine where you can see the actual market landscape at ClickBank.
I used the pay version of wordtracker for close to 5 years. I've switched full time to the keyword tool within Google Adwords, it's just a better tool overall and it's free. I don't see the value in paying for wordtracker anymore.