You can make do with just a gambeson/arming coat and point the legs to it, but I've tried it, and found it much less comfortable. Since the arming coat sits away from your body and moves when you move, the legs are much more "floppy and free" just attached to the gambeson; attaching it to the inside of the gambeson helps, but still isn't optimal.
What seems to be the ideal for a white harness ("full plate") suit is to have the pourpoint underneath. It's a very close-fitted vest (as you posted the picture of), and the legs point to it - if your legs point in two places (front and sides) it works MUCH better. I've worn the legs to my harness a number of times trying this out. I've found that the pourpoint works much better underneath the arming coat, as it stays closer to your body and can be worn more snugly. The straps on the leg armor that go around your legs and knees really don't do much except for keep the armor from gapping away from your body during movement; the weight is all supported from the points at the top, and as Sir Edward said, the location of the points is very important.
Over top of the pourpoint, you have your arming coat. This is pointed for whatever else needed; spaulders, voiders, arms, etc. Gothic armor tends to be attached with points, and Italian armor tends to be attached more by straps; the pauldrons will strap to the gorget and will buckle around the arms; the arms themselves will be closed upper and lower and buckled inside the elbow, if fitted properly they won't move around even without pointing. In that case, the arming coat is there simply for padding (and possibly voiders if you use any), as the pourpoint is not padded at all.
If you haven't seen this (
http://www.selohaar.org/essays/arming.htm), it explains things very well, and is based off of period documentation.
