This mini-set was one of my first S+ purchases and it really helped me get a feel for which versions of this series were really "for me" (in my case, it's Froggy all day!!).
Having one of each gave me a much better feel for the subtle differences in what I consider to be the "optimal settings" of application for each different subset, and although I don't reach for the main MiG or Door Key oils as often as I do for Froggy, I *love* that I have enough of each to bust out the right Mercurial tool for whatever the situation is calling for.
My experience is that the set encompasses a range of specificity - i.e. if you know exactly what you want and need help getting it, go for Door Key. If you only have a general direction but aren't sure of all the specifics of what you want (or how to get there), Froggy is your guy. And if the destination is not as much of a concern as the journey, go for the main series, which of course helps with all things Mercurial, but in a less predictable way - at least for me. TL;DR - you get great bang for your buck with this combo and they're all fun in different ways 🙂