This.

If you're actually building a product to sell people it behooves you to put some effort into deviating from the base styles.

On the other hand if you're like me and use it every so often for a one-off project that you have no plan on monetizing then why not? I don't particularly care about the UI look and feel, I just don't want it to look awful which it would if I were to design it myself. Bootstrap gives me a decent looking sane UI with guidelines for me to follow to keep it all sane. Other than that, I just want to work on functionality.