Unlike grepping your BibTeX file, Zotero provides a nice tabbed PDF viewer, useful automatic import options with file renaming, and saved dynamic searches. Those are the main reasons why I still use it -- it's a good tool to manage my BibTeX file via a nice GUI, and faster than manually grepping BibTeX from the CLI.

Also, as my unenlightened colleagues insist on using ridiculous tools like MS Word to author papers in, it's useful to have Zotero installed for the ability to link a BibTeX file to a WYSIWYG word processor when collaborating. Zotero helps you play well with others.

I don't think Zotero is perfect by any stretch, but I'm glad that at least it allowed academia to reject both Endnote and Mendeley. Anything that punches Elsevier in the face gets a tick in my book.