I asked this same question to a gentleman that had worked with constructing a a couple B&Ms in the US, and this is what he had to say. There were two main factors in the origin of the B&M pre-drop:
1. Evacuations. Supposing a train crested the hill and balanced prior to going over because the chain stopped, getting people out of those first few cars really is quite inconvenient. If you have a flat area with a catwalk under it, getting those first two rows unloaded becomes much easier, than with the modern straight drops.
2. Perhaps the biggest reason was having to do with chain wear and tear. You'll notice B&Ms all have "feeder" wheels at the base of their lift. This is designed to match the speed of the train with the speed of the lift chain, so that when they engage, it is one fluid motion. The result is less stress on the chain. The pre-drop was implemented at the top of the ride to once again reduce tension and wear on the chain when the train began to disengage. After the first chain dog on the train has disengaged, there still may be one or two more still lodged in place, and the pre-drop made it so that for the most part, all of the chain dogs release from the chain at the same speed. These two features significantly reduce the amount of strain placed on the chain, and as such increases reliability.
(By contrast, I might point out, that most all other coaster companies do not have these design features on their lift hills. Whereas on most coasters, a cracked chain is not unheard of, B&Ms have a stellar reputation- their chains hardly ever break.)
Ok, anyway so why did they get rid of the Pre-Drop?
From what I understand, there is a simple, "duh", solution that solves both of the problems stated above, that was simply looked over in the design of the first B&Ms: Speed up the chain as the train starts to crest over the hill! A proximity sensor detects when the first car has reached just slightly passed the apex of the lift, and signals the lift motor to speed up. The end result is that the dogs release from the chain because the lift hill speeds up to match the speed of the train cresting and falling down the first drop. A keen eye will see on Nitro and Hydra that the chain speeds up as the train starts rolling off the top of the ride.
And because the chain motor is speeding up, it gives that extra little "nudge" to the back cars just in case the power were to suddenly kick out, so that there is enough momentum to push the train all the way over the lift hill, thus preventing any chance of the train balancing before the first drop.
Simply speeding up the lift chain at the right time relieves the necessity of a catwalk for evacuations, and unneeded stress on the chain. So I hope this solves your mystery!