I'm sorry, but you lost me there...?
Lockback: Buck had a titanium 110 a few years ago, for example. And Spyderco made a titanium Police, but i think that was a rather special run. Lockbacks are usually either in the lower price class of folders (where titanium is too expensive), or the very expensive custom knives by makers like Ron Lake, where titanium does not look sufficiently traditional. There are relatively few lockbacks in the middle tier, where the knives are expensive and "tactical" enough for titanium. I think that's unfortunate, and I'm happy that Spyderco is an exception and uses lockbacks in more designs than most makers.
Linerlock: Spyderco's Peter Herbst and the Bob Lum Tanto folder mentioned in this thread, for example. Many, many custom knives, too.
Well, people want titanium, titanium is seen as cool, it's the stuff they use to make bombers and submarines. As smcfalls13 wrote, gadget appeal and bragging rights. Look at all those knives with actual titanium blades, how many people have a real use for those? Someone has even started making titanium wallets - I doubt that this is very practical, but I'm sure it will sell. (And before someone thinks I'm just ridiculing others, rest assured that I'm a victim to the titanium hype myself - knives, watch, sunglasses, fountain pen,... all ti.

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Kristofer