Plymouth does work with the proprietary drivers - or at least it should. You generally won't get the same native resolution bootsplash that you'll get with a kms driver, but the bootsplash should work. You can even get a higher-resolution bootsplash with answers from this question. We could conceivably detect that sort of thing by default, but it's fiddly and likely to break.

Regardless of the prettiness of the bootsplash, plymouth is necessary to multiplex the boot messages that happen in parallel during the event-based boot.