We are talking apples vs oranges here. The printer that I use is a Monoprice 113860 Maker Select 3D Printer v2. It is a rebadged Wanhao Duplicator i3. This printer has been on the market now for several years and because of its low price, open source roots, easy modification and community support is one of the most prolific 3-D printers on the market. They are currently $269 on Amazon but I got ours for less than $200 on a Black Friday special a couple of years ago.
The Roland MDX-40 is somewhere in the neighborhood of $8000, approximately 40x more than what I spent on my Monoprice Maker Select printer. Even the 6040 4axis Chinese machines that I pine after are typically only around a thousand dollars. And the software that I use Autodesk Fusion 360 is licensed for free to students and hobbyists and the other package Autodesk 123D Design is given away for free by Autodesk on Amazon. https://www.amazon.com/gp/product/B01HE5O9IM/ref=od_aui_d_detailpage?ie=UTF8&psc=1
Despite the extremely low investment... I have produced some very interesting and/or amusing parts and devices. This is why I refer to it as one of my most useful tools. Other than a small space savings I no longer see any real advantage to a dual purpose machine. And I didn't even mention the biggest incompatibility.
A 3-D printer is an additive machine that basically makes no mess and can be used in an office or bedroom. Because the jobs can often take many hours they work best if they are kept very clean.
A milling machine depending on what material you are working with can be extremely messy and typically is much better suited for a shop type of environment. The time it takes to perform a milling operation is typically much less than what it takes to print a 3d part so an operator can stand by and clear the chips or shavings and observe the operation.
I used to use a manual engraving machine with templates in the house with brass and aluminum and even it was messy enough that my wife would sometimes have a fit. It was a small desktop machine about the same size as my small manual key duplicating machine which she also insists that I use in my shop area.
The thing that keeps her on my side is that she is a fairly well known historian and lecturer and I make her replicas of hard to find historic pieces that she is able to use in her displays at museums and events. My initial machining experience beyond school shop classes was eight years working as a millwright in a small lumber mill that my family owned. Environmentalists, the economy other factors and interests forced a change of occupation in my late 20s.
Which is why the business bought it rather than me. I mentioned it more as an indicator of what can be done with a physically light-duty CNC machine.
Yeah, I've drooled over the 6040 stuff, but I keep hoping for something better. If I could find a dual-purpose unit for ~$2K, I'd probably spring for it.