Ple-Sen
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I don't really remember, did mounts make much of a difference in past expeditions?
As far as I recall, they were only present in any significant form in Selenia. I even considered going as a rider myself. Of course, unless you count vehicles in RI3 as mounts, otherwise they mattered there too.
It's hard to say how it will be in the actual game. Since you can't resurrect horses, they might not be as imbalanced as they were in the tournament. However, this won't affect NPC enemies, and they'll be nasty regardless of the situation.