My recollection of the start of ICUS was in the days where you needed 500 hours pilot in command time (no cheating on that) to be issued a First Class ATPL. That was easy enough for GA pilots doing charter or flying instructors doing instructing. But the then Qantas cadet scheme meant a first officer joining as a cadet could log many thousands of hours F/O on Super Constellations and 707's when the seniority came up for a command. But they were stymied with the ATPL 500 command hours requirement. In those days it could be 15 years before your seniority number came up.
Qantas then used a DC3 and an HS 125 and after suitable training the highly experienced (by now) first officers would be authorised to fly in command on these types until reaching the magic 500 command hours.
Obviously this was a mighty expensive thing for Qantas. Horse trading commenced with DCA and the ICUS logging was born and now the command hours for an ATPL is very little compared to what it was originally. Later it came into the GA world (so I believe) when a pilot had to have ten hours in command on type before being authorised for charter work. I think it was then that somehow ten hours of ICUS was substituted.
Now ICUS seems to be the new "command". I believe in UK, where ICUS is authorised, the proviso is that if the real captain supervising the ICUS "captain" has reason to disagree with any operational decision of the ICUS chappie, then the ICUS is cancelled for the whole flight and co-pilot time logged. Needless to say this doesn't bode well for cockpit harmony and for that reason probably rarely happens. After all, the only witness to a disagreement is the CVR.