It's not a few megabytes, it's a few hundred to a thousand megabytes saved, on average. Multiplied by a thousand containers, and much larger layers on build servers, plus bandwidth, it makes a difference.
Worst case for slower processes, things take longer. Worst case for more disk use, things start crashing. For general cases, the former is preferable.
This assumes everyone is on exactly the same version. Larger organizations can afford to appoint SREs who can institute a broad range of security and optimization policies (including "all apps should use the same Alpine base image version") and enforce them programmatically as well as ensure that apps are continually updated to match them. That kind of thing is expensive, resource-wise, for smaller organizations.
I'm not talking about image waste, I'm talking a single box with a half dozen different base image versions and a slew of extra packages thrown in. Every app built against glibc is going to get bigger, and not all Ubuntu packages are built with small size in mind. Compare these systems with Ubuntu vs Alpine base images and the average size for an Ubuntu ecosystem is substantially larger.
Worst case for slower processes, things take longer. Worst case for more disk use, things start crashing. For general cases, the former is preferable.