for several reasons
1) there are lots of fakes that you wont spot if they have the same name,structure etc on nzbindex
"Looking" inside archives is a HUGE +
2) Mysterbin SEARCHES INSIDE archives, not only shows you their content(like partially binsearch)
3)Plus,this way you can search without even typing a name. Just using plain size filter and see what is inside those criptic archives
2) nzbindex doesnt have stuff (taken down or just because of technical glitches) that other indexers have.
same might apply to other sites.
Thats why if you havent found something via one indexer you might wanna try another one
Bookmarks