See this thread for the context of this post.
Hopefully I can help explain the method behind the madness of the 2 clicks and what the different modes do. I have talked about this a bit in the past (will update with link once I find the thread...). But, here goes a complete explanation using milling as an example (it's the exact same with prospecting and even disenchanting).
There is two parts to milling a set of herbs (by set I mean 5 of an herb - 1 mill's worth). First you have to cast the spell (/cast Milling), and then you have to "use" the item (/use Whiptail). The traditional macro and every addon I've seen works the same way. They do both steps in the same instant. This is definitely the most reliable and easy way to do milling but is definitely not the fastest.
There are a few things to note that effect milling. You can't do the /cast while the loot window is open. However, you can do the /cast before the loot window opens and the highlighted cursor which is waiting for you to /use some herbs won't go away when the loot window does open.
You can do the /use step while the loot window is open. What will happen is that IMMEDIATELY after the looting is complete, the next cast will begin. With the macro, there is some delay after the looting is complete before the next cast will begin. I'm not 100% sure myself what is causing this delay although I have a few theories but either way, it's there. So, this is what normal mode does. The button will enable for the /cast step once Milling is off cd and then once the loot window opens, the button will enable itself again for the /use step. This will give you a definite increase in overall milling speed and will work very consistently.
The "Faster" mode takes advantage of a very short window in which you can do the /cast and it will start casting while the loot window is open rather than just being delayed until the immediately after the loot window closes as normal mode does. This window is a few tenths of a second before the loot window opens. If you are too late, you get the same effect as with normal mode. If you are too early, the spell will get interrupted when the loot window opens. Just right and you will cut a very significant chuck of time off your milling. The button will enable for the /cast step when the cooldown is up just like normal mode. But. it will enable for the /use a arbitrary amount of time after the cast finishes. This amount of time is essentially just a guess on my part and will almost certainly be too early. However, if you can figure out a rhythm where you can get the /use right in that time window, you can train yourself to have a good sense of when to hit it.
In conclusion, I'd recommend using normal mode if you just want to spam the button while watching TV and get through with your milling / prospecting. I'd recommend using "Faster" mode if you'd like to get through the milling as fast as possible even if it takes some practice and requires more of your attention to pull off. Also, I would agree that having a mode that just does the typical old-fashioned macro approach might be a good idea for those who don't want to have to spam their buttons as much and don't care about speed.