Shifting is rare, but as you pointed out also stops lethal hazards.
Dextarius is a Titan, his original card just doesn't tell you that.
Good thinking looking to Echelon for an answer, but mounting Dextarius with Ywain would have actually prevented either of them from moving through other Legio XIII models, if Dextarius were not a Titan (Dextarius is neither Legio XIII nor a Gladiator).
Note that this also means that mounting the pair would prevent other Legio XIII models from moving through them with Echelon.
However This will be changed in an updated .PDF next week, and both models will benefit from each other's movement-related special rules.
Also 1" push or shift away should be treated as still within 1" (Mathematically hairy argument, but it is ruled as such
).