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what would the point of mechs with hands be?
Hands are on mechs for alot of very good reasons. They enable you to pick things up (supplies, obstructions, etc) they enable your mech to prevent falls by putting the hand out to brace so as to avoid the mechs head smashing into the ground (just like a normal person)
They enable a mech to get up of the ground once it has fallen.
They enable a mech to more readily assist another mech, to carry out field engineering/repairs...
They enable a mech to rescue pilots from the wreckage of other mechs, or to clear rubble off friendly mechs.
Alot of mechpilots prefer mechs with hands in the canon for the utility of them.
Customisation via changing weapons is non canon yes, and wont be in Mech Warrior Online, as you can see from this mech xp tree example:
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We all know that mech variants are the same mech with different weapon combinations and suchforth- so those unlockable variants would be pointless if you could swap weapons at will.
People assume that swapping weapons is the only way to customise, but the devs have already stated that they will be including module slots on the mechs to put role based customisation modules in. So the Canon should be more intact in MW:O than in any previous MW game.