A final point which is sometimes forgotten in the discussion of tourbillons in watches, is that the "single average rate for the vertical positions" is itself subject to instability. Having a single average rate is certainly a theoretical advantage, but that single rate is an average and itself subject to variation depending on the amount of power in the mainspring, the state of lubrication of the watch, and the amount of time it spends in varying positions where poise errors may be more or less exaggerated. The latter issue is the one that multi-axis wristwatch tourbillons are meant to address, and certainly, such tourbillons – those from Greubel-Forsey, for instance – remain fascinating as intellectual exercises. That may sound like damning with faint praise, but it isn't – the intellectual interest of such watches isn't for everyone (and certainly the price is not for every wallet) but they remain intriguing examples of the exploration of bleeding edge mechanical solutions to long term rate stability; experimental constant force escapements like Girard-Perregaux's are another example of such experimentation, and I think the watch world is definitely richer for such things.