I consider the various invasion events to be more multi-zone events since they can occur anywhere. Zone events are the smaller occurrences like the burning buildings in Steel Canyon, the Supa-Trolls in Skyway City, and giant monster appearance that are typically relegated to one zone like Babbage in Boomtown (except when summoned by the Synapse TF), Lusca in IP, Jurassik in Crey's Folly, etc
I don't think any of the zones events are wasted but I do think we need more of them, and maybe spread some of the smaller ones out to other zones. Fires should happen in more places than Steel Canyon, but have different enemy groups behind them. Zombie apocalypse is fun, but maybe occasional smaller zombie uprisings in zones that have graveyards, like Striga, Dark Astoria, and Sharkhead.