Plus, whats the point of Elementium, when the armor it creates isn't even the best, or even close? Heavy Master armor is far better, and all you need for that is a bunch of iron. Would be a lot cooler if this rare element could be produced to make something really good.
I could be wrong, but I believe the Legendary armour has no combat speed penalty. Granted the penalty isn't huge for a full plate suit (it's what, -0.1 for each piece? So with arms, legs, body, shield and helmet it's just -0.5, not a huge penalty), but if you had a force of heavy soldiers armed with full Legendary armour, equipped with swords, steeds, and items that increased combat speed, they would be VERY deadly.
Also, since Legendary armour comes from the magic/sorcery research trees (I believe, I may be incorrect), it would actually be easy to get that armour a lot quicker that the very high level mundane armour.
Edit:Additional: The only real use I get out of Legendary armour at the moment is buying it at the item shop and equipping my Sovereign with it. Makes him stand out a bit from the rank and file.