When Mods find ones that have been already implemented, they move them out.
Although, it would be hard to determine what doesn't make any sense. Unless written in utter gibberish, a suggestion could make sense to many people but not those who are savvy in area. While it wouldn't make sense to the savvy people, it could lead to other ideas to implement something that is along those lines, but better.
To me, it would make sense so that potential buyers could see how much load is being produced on this site. Therefor they can compare it to the load they have on their site and see the difference.
However, in doing so, XenForo team would have to mask some areas out, due to potential security risks (and it's doubtful that they would ever do that)
I would say ones like that example are quite few and far between; there are plenty that might be niche suggestions, but they're still valid. That forum is always going to grow, unless you just arbitrarily move suggestions out -- but based on what criteria, and where do they get moved to?