Uh, no, I know how to draw, and I think SAI is limited because a.) it lacks a text tool and certain shape tools (there is no circle tool, and unless you are da Vinci and you can draw a perfect circle freehanded, then a circle tool is a nice thing to have), b.) it lacks many of the layer and blending modes available in, say, Manga Studio/Clip Studio Paint and c.) There are no perspective rulers, which are immensely helpful, and while I know it's perfectly possible to draw good perspectives without those tools, they are extremely useful tools if you can get them, and d.) its magic wand tool is not the greatest.
A better, larger program won't teach you how to draw, that's true - but SAI is lightweight, especially if you're going to draw comics. And I say this as someone who has completed professional illustration jobs using only SAI, because that's what was available to me at the time.
I will give you that it does amazing lines, especially compared to Photoshop - but compare it to Manga Studio/Clip Studio, and it comes up kind of short.