I have no idea why i was mentioned lol....
Judging by what you wrote, you are saying some APC/Tank with thin armor would be suicidal to use in an armoured engagement??
The aspect of a sole tank regiment/division is a thing of the past. Today, you need to use a combine set of force to augment any attacking force to try and suit every purpose.
In the US, we use a combine force doctrine. Which mean Infantry division would have both Cavalry and Armored Regiment mixed in it and Armored Division would have mechanised Infantry mixed in. The lighter vehicle would do their job well too if they were accompany by larger tank or what not, and we need those thin armored or aprtially armored vehicle to bring troops in, recon the area and fill the rank of whatever the big boys need.
I used to be a Bradley Commander, the fact that my bradley may encounter a T-72 is thrilling and scary, but without my bradley and the 6 troop in my compartment are more important to stop gap the enemy sneaking attack to our armor, if they can sneak within a line of tank, even a sinlge soldier with a RPG-7 can deal hack a lot of problem. The advantage in this case out weight the risk a lot.
I don't know if this is what you ask, hope this help