Digital clusters can be a good thing if they're used to provide more information to the driver. The digital clusters in the new Audi TT and Lamborghini Huracan can display the navigation in addition to the speedo/tach directly in front of the driver:
If you don't want the map full screen, the display can be configured split screen, or with the map in the center and the gauges on each side if you like.
And in the future, automakers could eliminate the side mirrors entirely and simply project the image from the side camera in the digital cluster whenever a car enters your blind spot.
So lots of potentially neat ways that the digital cluster could be beneficial for drivers. But if automakers just use it to display the tach/speedo digitally I agree that it doesn't make much sense.
BMW uses the same NVIDIA graphics chips as Audi/Lamborghini in their instrument clusters/infotainment system, so the hardware is there if BMW wants to take advantage of it. It's merely a programming issue, and of course they'd need to integrate a better display for the cluster. Hence why an OLED panel would be ideal.