I always add a combination cable to my bikes with good saddles -- and that's all my bikes -- as a theft deterrent. I did the same on my Brompton. Is it overkill? I was thinking yes, because you have to unbolt the saddle and pull the seatpost out from the bottom (which a thief would not bother doing, because the saddle is the prize).
The cable is not exactly hurting anything, other than looking slightly ugly. I do tend to carry a u-lock and cable (Kryptonite), so I could always use that much heavier cable if I actually had to lock the bike.
Just call me oLynne. iLynne has left the building.