There must be. I've made blueprints that were just made of trees.
Unless I found some kinda bug.
Sure, you can make scenery groups with nothing but ungridded parts, save them as blueprints, and plunk them down as units. But if there isn't a gridded part in the mix, it all ceases to be a unit as soon as you deselect it. At that point, it dissolves into its individual bits of scenery. You can no longer select more than 1 piece of it at once.
This isn't really an issue with groups of trees, rocks, and bushes. You can't recolor them anyway and usually don't want to rearrange them too much. But suppose you made something that you want to multiple examples of, each of a different color, or some sort of manikin that you want in multiple poses. To do this easily, you want to be able group select multiple parts at once, either to recolor or reposition. However, group select really only works well if the thing you're editing is treated as a "building", which requires it to have at least 1 gridded part. So if you don't have a gridded part, you're reduced to selecting and recoloring/moving each individual part 1 at a time.
For instance, I made an alligator out of nothing but non-gridded art shapes. But I started with a gridded flat roof panel lying on the ground, then built the alligator about 8m in the air above the roof panel. Thus, the alligator is treated as a "building". I can put the roof tile on the bottom of the pond where it's invisible, adjust the alligator to be on the water's surface, then group select all the parts that make up the alligator, copy them, and put the copy in a different position and orientation. Or I can select some parts of the alligator (like a section of its tail) and bend them relative to the rest, to get different poses. This is possible because the alligator is a "building". If it didn't have the gridded roof piece, then I could only affect 1 small piece of an alligator at once.
