We want to be sure that the results we show in the cross-wiki search results side bar are actually relevant from each project.
For example, if I search for 'pants' on Wikipedia, we don't want to show some obscure results from Wikispecies that might have the word 'pants' somewhere in the description (like 'cats wearing pants' or 'beetle crawling up my pants')
If there aren't relevant search results to show from any number of the sister projects, we shouldn't show the non-relevant ones just to have that project represented.
We just need to figure out a great way to determine relevancy for the sister project search results. Easy, right? ;)