As the tutorial says.... duplicating is one of the most common operations. In illustrator, I had the habit of doing the following which turned out to be quite useful: select something, then alt-drag it... it'll be duplicating.
There are only 3 modifier keys (Ctrl, Shift, Alt). When dragging, all of them are now occupied with things that are directly related to dragging. We cannot spare one for an action which is strictly speaking unrelated to drag (copying). And anyway, I don't see how "alt+drag" is more convenient than "ctrl+d, drag" - it's just one keypress less.
Actually several other places with a duplication "modes" exist. For example all transformations have a "copy" checkbox...
That's particularly nifty when combined with Illustrator's "repeat last command". For example one can easily rotate by 45 degrees with copy, then request repeat-last-command 6 more times to get the objects spread around in all 8th of the circle... Making a horizontal sequence of strokes is another example.
That one is indeed very useful. I thought about that. Please search our list of RFEs and if you don't find it, submit a new one.