The Macbook Pro still has the ethernet port. The retina does not. If you need a Macbook pro and need the Ethernet port BUILT IN, get the standard Pro.
The retina is a new line of laptops. How long it will be before they no longer sell the Macbook Pro without retina? Who knows. They said so in the announcement video. With it being a new product line, they were able to eliminate the ethernet port and superdrive.
For YOU. Apple gave you a choice. It is a nice mobility and slim design. If you need the ethernet port, get the adapter. If you dont, you have a nice slim mobile powerhouse at your disposal.
I think that for the most part it is safe to say that the people who have a problem with this weren't really considering buying a Mac anyway. I'm sure that there are folks who were considering a retina MBP but are put off by the lack of the ethernet port (but I agree with you that it is silly and there is a perfect fine adapter for it).
The underlying issue is that when it comes to Windows laptops, there are a TON of OEMs who each output a TON of different laptop models. So if a particular Windows laptop doesn't have a particular feature, there are a lot of choices around that. I don't want Apple to operate this way. I like that their hardware is razor sharp because they've put everything into the small set of models that they put out there. This is their edge, and Apple fans love that edge.
I'm looking at my ThinkPad at work and some of the design choices are baffling to me. For instance, the video output is a DisplayPort. Even though, the vast majority of "DisplayPort" monitors actually use Mini DisplayPort connectors when my laptop was released (2011). Ok...you don't see me throwing baby fits over the fact that ThinkPad didn't include a DisplayPort to Mini DisplayPort adapter. No, I just bought the adapter because complaining about spending an extra $20 when the laptop cost $1800 just doesn't make sense. Windows OEMs don't include these adapters, but when it comes to Apple a lot of people just want to cause a stink. Furthermore, almost all Windows laptops (my ThinkPad included) place these random ports all around the laptop...I have a docking station at work (which I love btw), but at home after I connect the power adapter in the back, the display port on the left side, and the ethernet port on the right side the thing just looks ridiculous. Why would you want these ports all around the laptop when having them all on the same side (like the MBP) makes wiring look so much cleaner? Its these little things that the Windows laptop OEM guys do that makes me wonder if any of them actually use their own products.