I actually was thinking about the same question as well for the last week or so. The main reason IMO for a HM oil would be for leaks/seepage additive like in Maxlife or Mobil 1 HM. Plus, these are higher viscosity oils than their non HM couterparts, which would be better for HM vehicles, too. An SL level of additives would be welcome to me. I've tried the Mobil 1 HM and it works quite well, both 10W-30 and the 10W-40. I think Mobil has a winner there. OTOH, if it doesn't sell well, they'll be sure to discontinue it.
Amsoil makes an oil for everything automotive, it seems. The only manufacturer with more varieties might be Motul. At this point, a few more products should not make a difference. Amsoil may watch and see how Mobil's product does before they jump in there.