Google will never completely filter out a whole extension unless you request it to, ie
this search. That search is specifically searching for .net sites only.
extensions are good for localizing your community, such if you're in the EU, a good idea to have a .eu extension. I note, it's only a good *idea*, it really doesn't have much effect on SEO.
If I own a .eu domain and host it in the US, then Google thinks it's probably geared toward US visitors. Likewise, if I have a .com hosted in the EU, it's more likely geared towards EU visitors.
It's simple geo-targeting search.