I've been firmly pro-spaces ever since I discovered there was an everlasting war over this, and it came about primarily over documentation. Say you're writing documentation within a /***/ block, so each line is prefixed by three characters. Now say your documentation includes a code snippet. Or lets say that particular sections of the documentation (such as JavaDoc's @see) are indented so each line always starts after the @see. You end up with documentation indented with spaces because it's the only way to ensure consistency. And if you're doing it with your documentation... why not your code too?

However, my conviction has since been tested by Dart which opinionatedly forces you to use two-space indentation. There's no way to disable this and its IDE plugins enforce the style. I just find it so difficult to read, even with Rainbow Brackets. I yearn for Dart to use tabs just so I can configure the tabs to appear as four-space indentation. Or better yet, stop trying to coerce how people write their own code.