Go to Tankard of Tea for a fascinating discussion of the origins of IHS. Interesting from the perspectives of theology, linguistics, and art history.
E concludes with:
"It started as a clerical mistake. (Get it "clerical", as in, cleric, clergy, monks? haha) But it was perpetuated, I think, because of bad theology run amok. The middle ages were, shall we say, strong in venerating the holy and awesome side of God; in cultivating a fear of God and respect of our position as sinners. But we all know they went a bit overboard. I think they forgot that God Himself invites, nay, tells us to call Him father and brother, co-heir. He prepares a place for us in His eternal home, writes us in His book of life, invites us to dine with Him weekly, and made The Greatest Sacrifice for us. Surely we can speak His holy and most beautiful name, and must not confine ourselves to abbreviations. We know we are unworthy and unfaithful sinners, but it is the story of salvation that He says "Call me by my real name, anyway". Just like it is not our choice to exclude ourselves from His table when He calls us to come, it is not our choice, with mock humility, to call Him something other than what He wishes. Particularly when that something other is, to modern culture, so impersonal: an abbreviation or acrostic."
This is particularly noteworthy since it has become vogue to write G-d or some variation thereof for God. This not only is a corruption akin to the IHS controversy you describe, its lineage can more directly be traced to the Jewish tradition of dropping letters when writing God's name. Your discussion of why theologically, in the light of Christ's coming those traditions or fashions are not valid means of expression. Christ himself has removed the barrier, and we are adopted sons... to call our father, "Father". To do otherwise is to begin down the path of creating your own traditions that seem wiser to you than God's own demands.