It's only ridiculous if you don't understand the idea behind it. Feel free to make a derivative and share, I'm glad you like the shape.
When you shell the model out to 3mm, your slicer will need generate two or more "outer" walls (the inside and the outside of the vase itself, multiplied by the number of walls you specify), with, probably, infill between them. It is very difficult, if not impossible, to create a model one wall thick, and have it slice reliably with no infill artefacts.
Using the model as designed allows you to simply specify the number of walls needed. The slicer will generate no infill between them at all. For more advanced users (using, perhaps, more advanced slicers), this presents the opportunity to print the model as one continuous spiral of plastic (aside from the base), resulting in a far smoother print — which prints far more quickly than a normal layered, infilled, start-stop print.
I hope that helps you to understand the reasons behind this approach — and to see that it's not ridiculous at all, but rather, quite useful.