It could be that they ran out of encoding-bits left in the instruction word for that particular instruction or "instruction family" and thought it ugly to do a special separate instruction for that function. The encoding info in the programmer's manual could give a clue.
I should add that apart from a remade Amiga on steroids, I'm also all in favor of starting from the very base and get that perfect to build on that (Schoenfelt/A1k etc). Nothing wrong with that. But the ultimate dream would be to have a 100% compatible chipset and a completely remade but 100% compatible CPU at blinding speed (ie. super fast memory and running as fast as it can with excellent caches). That would open up some possibilities like fast web browsers and Javascript engines (HTML5 is the new black and Flash can go hide in shame, finally), and competent dev environments with nice high-level languages.
For other people of course, not me. I don't mind the limitations of the real deal, it's their challenge that is the reason for me to still be into Amigas.
Lordy, my English is starting to fail me, a clear signal that it's time for bed.