Asking candidates to complete problems at their leisure is a very good idea. But recruiters can bungle up on this too.
For something that's purely procedural in nature, asking for a complete and proper object oriented solution would not be such a good idea. Likewise, forcing the candidate to use pure procedural method for something that definitely needs OOP.
Fortunately we have good recruiters. Once they've identified a candidate they play purely a support role in the technical side of the evaluation process.
I'm not sure I follow that last statement 100%, but if you "go against the grain" all we'd care about is that you're able to reasonably articulate the "why" behind your decisions.
For something that's purely procedural in nature, asking for a complete and proper object oriented solution would not be such a good idea. Likewise, forcing the candidate to use pure procedural method for something that definitely needs OOP.