An unlined plastic handle is strong enough for most everyday cutting (string, boxes, packages, paper, etc).
But adding dual steel or titanium liners to that same knife will definitely increase its strength in many aspects.
And dual metal liners also allow for a greater selection for pocket-clip placement (right, left, tip-up, or tip-down).
Also, dual metal liners lend themselves to all-screw construction--and most guys like to tinker and disassemble their toys.
So you get greater overall strength, clip placement options, and the ability to disassemble--all at the cost of merely a few onces.
Why would anyone not want dual liners?
For me, the real question is: why dual steel or titanium liners and G-10?
FRN plastic slabs on titanium liners would work just fine and still keep the cost down.
Cheers,
Allen.