1. You can drop through the platform without having to jump, WD, or unshield (takes about the same amount of time as a WD). You drop through faster than an Isai drop. You can buffer the shield drop during landing lag to drop through asap. You cannot buffer a regular drop through to my knowledge, and you may lose frames by walking for slightly too long before dropping through during Isai drops.
2. Watch all of the first 4 Melee videos:
http://www.youtube.com/results?search_query=dashdancedan platforms&sm=3
You might also want to dig up Shai's thread on Shai dropping (should be pretty easy to Google for).
3. Shield dropping is meant for whatever you can find a use for. The most popular reasons to use it are speed, counterattacking opponents hitting you from below, escaping pressure from above, allowing you to DD on platforms while still being able to drop through, positioning yourself on the ground more accurately than if you could only run off the plat, and turning around before attacking.
4. Yes and yes.