In MW:O, every 'Mech and weight class is trying to be as viable as each other. Hence 'balance'.
In lore and TT, that wasn't the case. The way I saw it was that a light BattleMech was the equivalent of a light tank, effective and useful in some situations, especially for scouting or whatever, but ultimately a heavy or assault BattleMech was going to be worth multiple lights in a straight up fight. Plus economy. Lights cheaper and quicker to produce.
MW:O is... pretty much just a straight up fight, scouting is limited in usefulness and depth, and there's no economy. So PGI is trying to make taking, say, a Firestarter in a fire fight is just as useful as taking an Atlas. Just from that completely separates MW from lore. I was always kinda hoping PGI would take economy more seriously in CW, making light and mediums cheaper and easier to run while assaults much more expensive and tricky to maintain but obviously worth multiple lights in a battle. Instead we just got a new game mode.