Not always true, Some of the stiffer (non race specific) boots for next season are made with the new plastics like Grilamid. There are boots that are low and high volume based upon foot shape some will use the design to create room and others use it to make the boot more efficient. YMMV.
Mmm, I think you misunderstood me, I was talking about having two pairs of LINERS for the same pair of boots. You seem to be talking about shells. Or did I misunderstand you? If so, can you clarify?
What I was talking about was owning one pair of boot shells, with two pairs of liners:
If the liner is thicker it will create a snugger fit, and often be stiffer (Intution Prowrap for example) and warmer. At the same time a thinner liner in the same shell will be roomier (nice for hiking), and more flexible (especially if it’s a dedicated touring liner). With similar foams, a thinner liner is lighter and less warm, both beneficial for touring.