Adding the scenario selection screen into GB would change a lot of code. We had a hard time figuring out how exactly to add the map packs and there are a lot of limitations that we found with the current design. As this game is focused around modding and adding new content, we're learning new stuff as we go. Originally they were going to be released like mods, but then we wanted them to stand out more. So we added the additional battles screen, but then they are loaded all the time. No way to turn their content on and off. So that severely limited what we could do with them.
Although it is possible to install both on the machine, it is not easy given the way the installer works. It's searching for reg keys to see whether or not you have GB installed and then just defaults to map pack or stand alone based on what it finds. To install as stand alone if you already have gb installed, and I haven't tried this, you would have to rename the reg keys (matrix and norbsoftdev), then install. Once it's installed you would have to copy the new content from the reg keys under the old keys. Delete the new keys and rename the old keys back. Yes, I realize it's a hack, but it's something that we didn't foresee people wanting to do. Our main focus was on making sure that you could MP between the games.