This is a feature I find *really* desirable.
The main reason is it allows for easy classification and grouping.
I'm not yet very familiar with Silverstripe, but essentially the way I'd envisage something like this is that when you create a 'page' in the admin you also create a 'route' to it. By default that would be /category/sub-category/title - So in creating this forum the route would be set to, by default, /forums/feature-requests
In handling requests, you look for defined routes, which will direct the request to the relevant controller/action.
You can implement a system to reserve certain words perhaps? So setting up a subforum called 'Reply' or 'View' would not interfere with the actions for this forum. Instead they would have a forced prefix perhaps, or suffix.
Again, I'm not that familiar with SilverStripe specifically, but this seems like it should be a workable - and backward compatible - addition. Where no routes is defined for a request, it would fall back to default behaviour.
Another benefit of something like that would be the ability to have multiple URLs serving the same objects by simply declaring additional routes.
It really is a feature I think should not be overlooked. Even in designing static website I always use directory naming to provide logical grouping to URLs. If I have a hierarchical menu structure, I will aim to have URLs that reflect that structure.