I definately employ this tactic. The ruck situation can be used tactically by either the defending or attacking coach (suck in potential ball carriers or thin the defensive line).
Part of what I enjoy about CT is that you can play it high risk when attacking (forget about your defensive lines if you happen to turnover the ball) or keep it safe (hold your players back in their defensive line, which limits your attacking options).
Rucks are perhaps utilised in their cleverest form when forcing offside lines (regardless of if you are on attack or defence).
I am glad to hear this (my son, who is ten argued this with me), but the way the rule is written on the website strongly implies that only the team who had possession of the ball prior to the tackle was eligible to protect.