why is it that....

[Aquattro]

While "softies" do in fact use calcium, they in no way compete with an average SPS colony for Ca consumption. LPS may be real competition, as they do deposit a lot of Ca, it's just hard to tell because of the massive growth instead of branching.
On the coralline issue, again, no way.
None of this means a reactor isn't useful on a "mixed" tank, just that a reactor is often considered "required equipment" for larger SPS systems, hence the assumption that reactor purchasers keep SPS.