Do you find you use the half cup?
All the time. I mix my coffee, oatmeal etc in the small cup thus keeping the large one clean. I didn't think I would bother with it either but it is real handy. Saves so much space the way it slips over the bottom of the larger pot as now I don't need to carry an additional mug anywhere else.
What value does a lock-on lid provide?
I tend to carry things in it when not it use. As soon as it tips everything spills out because the lid does not stay on. If it did, it would also be way harder to crush as the lid would be adding that extra support. Without the lid on I can squish the mouth of it together by hand its so flimsy. I have to be very careful where I pack it so no pressure gets placed on the mouth of the pot because of that. Small point I know, but a real pain all the same.
If the lid locked even a little, it could also be used as a way to carry water if you had to.
Good warning on the Ti (especially when it's thin & light like this)!
Learned this lesson the hard way. When I upgrade, I may get the exact model in stainless for this reason alone.