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Since the cylinders are just attached to the bottom they can indeed break off more easily. However, they shouldn't come off that easily. So far, it rarely happened to me but then I didn't test the bricks with real kids. I didn't want to embed the cylinders into the roof since the roof already is difficult to print. However, one might think adding some lateral stabilizers like Lego does for Duplos. Actually there already is something like this, but it only will show up on DUPLOS or Legos printed with 0.1 resolution. Certainly could be improved.
No idea what Lego thinks. However, I carefully avoid calling the bricks "Lego", since it's a trademark. The bricks are called "Lugo" and "Doblo" or "lego/duplo-compatible". Since Lego is loosing every court case against products that are Lego-compatible, there is nothing to be afraid of. Finally, so far, printing a piece is more expensive than buying one. In addition people who print a few weird Lego pieces may buy other pieces, i.e. 3D printing may be good for their business.
This being said, despite loosing court cases, they always can try to threaten places like sulpteo or shapeways since the word lego could appear in product descriptions a bit too prominently. http://www.shapeways.com/search?q=legohttp://www.shapeways.com/searc...