> Any alert is better than no alert

For large earthquakes, absolutely. But if you get a situation like the Canterbury earthquakes (https://en.wikipedia.org/wiki/2010_Canterbury_earthquake - 17,000+ over the span of several years) the warning system becomes useless unless it can rapidly detect magnitude and mercalli intensity.

My family lived through the Canterbury earthquakes. If you live somewhere with this hazard PLEASE prepare accordingly.

Depending on your proximity to the epicenter, sometimes you can hear a big earthquake before it hits. It won't help much if it hits in the middle of the night when you're asleep, but a second or two is enough to drop under your desk and avoid the most frequent danger, falling objects.

In countries with decent engineering standards, broken glass and items falling from above such as light fittings and ceiling panels cause the worst injuries. Giving people a minute or so warning could be very helpful to prevent these sorts of injuries.