apart from lighter weight what are the benefits pray tell . . .?

That’s it really,

probably more flexible but also more likely to bend permanently too.

unless it’s the same weight in which case it will be stiffer and less likely to bend 🙂

but the diameter is pretty much restricted…

I am not sure I like the idea of hollow Ti rails due to the risk of crushing and bending around the clamp. But I guess they are only used by lightweight people who need the extra flexibility in the rails to get the same amount of flex as a heavier person.

I’ve had 2 Selle Italia SLR saddle rails crack on me. I think they’re ti (hollow). They were both on the same hardtail used mainly for road riding and gentle off-road (my winter bike). I weigh around 13.5 stone so I’m not super heavy. Wouldn’t buy another Selle Italia saddle.

They’re more flexible (pro or con) and less strong (con). And I’ve broken two saddles with hollow Ti rails in the last couple of years. I now have cromo rails…