Hey (: I asked my friend (first language japanese) when and how they would use 一本気 and they said I never used that ^^ So why would there be a reason to add it as a vocab in WK other than it uses 3 kanji that were already learned or are there any examples of how to use it that would be useful 
Firstly, WK is a tool for learning how to read kanji, so while there is often a lot of correlation between common words in spoken and written language, just because a word isn’t commonly used in conversation doesn’t mean there’s no value to learning it. JPDB lists it as appearing in about 5% of the novels and visual novels they track.
It’s generally safe to not assume that you should use a word in conversation if you haven’t checked.
That being said, it’s still not a particularly common word even in writing, so I would likely have recommended something else if I had a say.
WK does have some entries that feel like they were picked because the creators liked them or thought they sounded interesting etc, and did not foresee that learners have a tendency to want to use everything they learn.
Haha - thanx - my favourite part “did not foresee that learners have a tendency to want to use everything they learn.” So yes, I shall not try to use everything I have learned then (..◜ᴗ◝..)
I think this one was added as it was used to illustrate that rendaku can change the reading of a kanji, and that was the option they had with some of the very early kanji.
It’s funny, but 一本気 was one of the first WaniKani words I ended up recognizing in the wild. I was watching Hanasakeru Seishounen, and one character used it to describe someone else. You really never know which words are going to end up being useful!
definitely echoing @lilla_kertesz ! no effort is wasted 
if you only learn the most common words in any language, you’ll never cease to be shocked and annoyed at how commonly uncommon words pop out at you.