XML isn't as common as one would think. It's been steadily decreasing in popularity and use. It's a very verbose format that is suited to enrich a larger set of data, such as HTML documents. For data heavy documents where, it's a particularly bad match, as you end up using as much text for annotation as the data itself.

Using XML for 3MF is IMHO a technical cop-out, where you don't really want to solve it "correctly", so you go with something that is "good enough". With XML, you know it'll be able to encode anything, be human readable, and have existing parsing libraries in pretty much any programming language and standard libraries. So, it makes sense. However, if you're creating such a format, the least one should do, is write a sibling standard for how to directly binary encode the data. This isn't a hard thing to do. It just need a standard for how to do it, so everyone agrees. Here is an example online on how a rudimentary implementation could be done for OBJ files, but the principle is the same. That way you could chose to export either as 3MF or 3MFB (for binary), and as long as your slicer, and what not, can decode it, you're good.

The hard part of 3MF was all the great work in standardizing what, and how that is represented.

The sense I get is that it is more lazy than anything. The verbiage feels like the fact that designs were public documents was tacked on last minute to satisfy some desire for market segmentation or to create a parts and design library to draw traffic. It would make sense that the company hosting the software would not want the headache of being unable to use your stuff commercially or even of parsing what they could use, since in some sense they always are using everything commercially. Refusing the to thread the needle with their verbiage, though, has left a situation where the Terms of Use say clearly that (1) a design is Content, (2) a free user's Content is a public document, (3) a free user cannot use their own public documents for commercial use, and (3) a free user grants EVERY OTHER USER a license to sell their public documents.

1. "End Users’ files, designs, models... (collectively, “Content”)."
2. "All documents created by a Free Plan User, and all Content contained therein, is made public and therefore considered a Public Document."
3. "If you intend to use the Service outside a trial context to create and/or edit intellectual property for commercial purposes (including but not limited to developing designs that are intended to be commercialized and/or used in support of a commercial business), then you agree to upgrade to a paid subscription to the Service."
4. "For any Public Document owned by a Free Plan User... Customer grants a worldwide, royalty-free and non-exclusive license to any End User or third party accessing the Public Document to use the intellectual property contained in Customer’s Public Document without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Document, and to permit persons to whom the Document is made available to do the same."

The only possible wrinkle is that the ToU distinguish between a "Customer" and an "End User," so maybe you the customer can grant you the End User the same commercial rights that Joe the slightly shady CNC machinist in Peoria has when he downloads your widget to fabricate and sell. Something tells me that PTC's license compliance folks don't interpret things that way, though.