More of a manufacturers recommendation - the Ti's way softer than the treated splined part of the sprocket and with the larger forces placed on the axles in a smaller point (as the crank arms have a larger contact area so force spread less) makes the axle more prone to failure.
Saying that, I've only ever seen one 'smooshed' Ti axle from a spine drive sprocket, and that was on a bike that it's owner didn't maintain it (loose crank arms anyone?) Sprocket was still fine though
If you get a Tree Sprocket (and they are very nice) give the splines on the axle a little bit of a clean up with a wire brush (not too much though) as they're a snug fit
