Rather than make it an option - I implemented that it remembers the last state. Adding an option is not great because
1. People don't realize that the option is available
2. The default will be to always show it, and based on #1, no one will know how to make it do what they want, and hence will always be frustrated, and maybe, just maybe, they will post their frustration, and we will reply and tell them how to turn it on, and then and only then will be happy (but after days/weeks/months of frustration).
By making it a passive option, i.e. it just automagically remembers how you last use it, it just works!
I assume you are just floating your mouse cursor over the collapsed toolbox tab to get it to show and to use it. You are NOT pinning it and unpinning it.