>While there are valid reasons to ask such questions they need to be framed carefully as to indicate why the developer would want to do so. Failing to do that causes frustration because in general the first rule is don't re-implement the standard library.

Seriously, you're bothered because someone asked you to solve a toy problem? Most interview problems are things one wouldn't do in the real job; that's because all real job problems take more than an hour to solve.

Implementing a stringbuilder is a bad question because there's not much depth to it (after you get the basic implementation down, where do you take the question next?), not because there's a stringbuilder in the stdlib.