I assume the market for these are people who weren't around when these classic systems came out, and they want to connect them to an HDMI TV. Since this market may not have experienced CRT technology they might feel there are no drawbacks with this solution.
For me, personally, outputting to HDMI from a classic game console is just not what I want to do.
Last week, I bought a complete GEN 1 HD Graphics Sega Genesis (Mega Drive) with original controller for $15USD at a retro game store. It came with the cables to connect it and it works fine. I had never owned a Genesis/Mega Drive back in the day, although did rent one when they first came out with Altered Beast. Playing through Sonic for the first time is a lot of fun. I played a good bit of Sega Master System. For me, I think all the negative ads Sega did towards Nintendo really turned me off back in the day. I enjoyed my Nintendo and didn't like a company telling me my original NES was bad. When Castlevania III came out, and Super Mario Bros 3 came out, buying a Genesis was a tough sell for me also. Now in this time, without bias, it's fun to experience more of the Genesis/Mega Drive library because there are some really good games. The feeling of the Genesis/Mega Drive is different than the SuperNES as there is a sense of smoothness of scrolling and fast motion that is not usually a part of SuperNES games.