Why should the Long Spear be purely a support weapon?
Well its now just a shorter, faster, and more damaging pike now. Can still whip it around and do some crazy stabs with it but the ground bumping has cut that down a bit.
As to why it should only be a support weapon. Honestly have no answer to this for you, the devs decided it should be so. Like I said its basically like a pike: Anti-cav and infantry support, though the Long spear does the infantry support a little better.