Oh yeah, definitely variations on a theme. If you look at car lines, like Dodge or Nissan, there are certain unifying aspects of their designs, but each model is its on piece. I know why GW chooses to do what it does. It is easier to throw in an extra $.30 of plastic, price it up another $10, and call it a day. If everyone buys it, why bother making a second line just for the one transport? If I were to do it though, I love the old wave serpent concept, and I like the general design of the modern Eldar (and then I've added a few ideas).
Long and short of it, I think we agree here.