Probably never.C4 is just too easy to grief with, and Dynacord Stick is Grenadier's Uber-Dream.
please bring back demo 3!! C4 is super legit and no one woukd ever dare to use it for griefing (and if they did so what, it's all fun and games) besides it's really awesome to blow up gangbangers in those silly town disputes
Also, the trap could be spotted, so it would make sense to invest some skillpoints in Traps for other players.
I have used the discussed tactic of setting up "traps" with c4. And it was quite effective and well down right unexpected. As a side note, by reducing the dmg of bombs detonated from the inventory would just be a cruel limitation on a fews favourite pass time. BRING BACK C4