You are correct, a weed border is nothing more than a cut line drawn around the objects' outer dimensions. You could easily do it manually. The Weed border can, however, be assigned to a keyboard short cut, and is intended to cut down on time consuming tasks. Draw square, size it, eyeball it, re-size it, etc... Weed border makes the square accurately on the first go.
Ultimately you can use what ever works best for your workflow.
The power weed is for putting weed lines "through" text and other intricate or difficult to weed objects. I type "through" since the weed line does not cut into the text, and you can adjust the offset of the horizontal line from the text.
I suspect that once you get used to using the weed and power weed, they will become second nature to use. Again, it all comes down to user preference.