The reason I sometimes use this facility with the GKP is to find volume amounts for the keywords I’ve chosen. But, these days, it’s not so important. As long as the Long Tail Keyword makes sense and is something I would put into a search engine, then it’s good enough. Sometimes the most blazingly obvious keywords are often over looked by eager, trying-to-be-clever marketers. Sometimes you need a tool to highlight them.
Given you have a good idea of where to start and are fairly confident you are speaking the same language as your client, jump start research by generating related keyphrases and long tail variants with the ever so easy to use Google Autocomplete. This tool makes predictions based on what you are typing that are a reflection of Google search activity.
The limit on manual keywords could be higher. I personally wish we could input 10,000 keywords at a time, instead of 200. However, I understand the costs that Long Tail Pro has to maintain each time a new manual keyword is input. Not a deal breaker, I just wish the limit was higher. (To be fair, I don't think most keyword tools have a bulk manual option at all).