This is an important question to answer according to me.Very Recently the alpha version of Bootstrap was released.Since then people are asking that...
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It's ok to know the bootstrap basics, but when working on a company you'll receive designs to implement, and those designs will not fit into bootstrap components, and may not fit into bootstrap grid system.
Also loading the entire bootstrap framework and it's dependencies inside your project affects performance.
The best option is to learn Flex Box and optionally CSS Grid and pseudo-selectors so you'll end with reusable components easily handmade with CSS only that fit exactly your needs, nothing more.
I totally agree with you but suppose you are creating a simple site you can use bootstrap and I know that Industry level projects cannot be created with bootstrap.
It's easier if you don't know how to use flex and/or grid properly but that's all. If you know those properties well there's no much point on relying into 12 column pre-defined structure
Yes bro