It does partially solve the issue of continuing to send messages without any sort of warning -- new devices mean your messages won't send unless you hit the "send anyway" button (and in your settings you can disable the send anyway option).

It also solves a problem that Keybase has which is that all new devices are automatically trusted (so a compromised device can just register more devices to avoid being blocked). To be fair, this makes for a pretty bad UX -- and the Matrix folks are working on cross-signing to make this easier -- but it is very useful for a user to be able to detect new devices to be detected (especially their own).