The major benefit of the method I use is that you get full aerial control in the direction you're holding. This is insanely useful, especially for Marth, as going straight down after the drop will get you untippered aerials. In the rare scenarios where you want to go straight down with your aerial movement, instead of full left or full right, just double tap fastfall after the drop.
Also, though I haven't tested it, I'm positive this method drops on the first available frame ... there is no way its dropping on frame 5-6 like the straight down method (effectively) is ... So I think its the most practical way of getting frame perfect drops. Definitely requires testing though, as I don't think any of us are sure -why- it works; if you don't hit the red "drop" area, why do you drop?
Another important note is that you can put your shield up in the neutral and slowly move to full left or full right and then do the drop, without rolling. Its generally better to have it held the moment you put up your shield, but if your stuck in neutral, and you want to drop without getting shield stun first, its still possible. I'm not sure exactly how fast (in frames) you can go full left or full right without rolling, but I've gotten pretty fast with it now (would be interesting to know!); it can't be more than a few frames. If you are already holding a direction (let's say right), and want to drift fully left after your drop, just release the control stick back to the neutral, and then slowly move all the way left. That ones a little more time consuming.
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To answer d z's question: dropping from a downward angle shield is interesting. Like Kadano said, simply holding the down angle direction morphs the red zone for a few frames. I'm not sure how this would affect the practicality of either method, but I have a feeling it lowers the consistency of the straight down method, since it relies on the red zone, and you can never be sure just how big the zone is at any one moment.
What I do is downward angle -> shieldstun -> go back to neutral -> hold left/right -> quartercircle down. For whatever reason, going back to neutral increases my consistency a lot. Its still really difficult, but I'm already decent at it, and I'm confident that with practice, its totally doable and realistic.