Joost developed his own keyword research tool to come up with keywords as well! Yoast Suggests uses the Google Suggest functionality you know from searching in Google. It finds the keyword expansions Google gives and then requests more of them. So if you type ‘example‘, it’ll also give you the expansions for ‘example a…’ till ‘example z…’ etc. Just go on and try it and fill out some of your potential keywords. It’s a great way to quickly find more long tail keywords you can focus on.
It would be great if you can provide comparative analysis of this tool with Google Keyword Planner. Since you’re using this tool so you can provide best comparison. One differentiation which I understood is that this tool works for yahoo and bing as well whereas Google keyword planner works only for Google but Google being a dominating search engine, gives the highest returns, so only this reason may not be enough to buy it.