I think that even with less stoicsm heavy tanks like hero/war/arms still more favorite with all their ability over a light tank. Only advance they have is the charge option. But on grp utility or survival abilitys all the advance on heavy tanks.
Bm has CD anytimer 0,96 and merc has DW anytimer 0,99. But zerk LA anytimer is even only 0,75. And on top of that merc has best absorb armor and both of them can use shield.
Here my solution:
1. light tanks get 50% of Memories of War and Scars of Battle > this makes them more durable but still weaker than heavy tanks
2. Zerk get shield option and anytimer LA from 0,75 to somewhere 0,9x
I think with this adjustment we get a better chance to see more zerks/bms or mercs on 8v8 setups