The RMB is a wonderful design, but what can you do on it you can't do on an iPad I wonder (iPad being even more portable) RMB seems to me to be a bit pointless in this regard
You've got an iPad that can split screen pdf and note taking software side by side, can run Scrivener and DevonThink and Tinderbox and Vim and Emacs and Xcode, screen share with the computer upstairs and can fit comfortably on your lap while touch typing on a decent keyboard?
I have an iPad and a bluetooth keyboard. It's OK for casual use (a bit of typing now and then, reading a PDF now and then) but it's a very limited and frustrating experience for my needs compared to the Macbook. The two don't even come close in usability for me -- in fact I'm considering selling the iPad because I don't use it much any more.
My Macbook is more powerful (for most of my uses) and nicer to use than the mid-2010 MBP 17" I also have. It's an excellent second computer and I couldn't be more pleased with it.
I wouldn't get the MBA because it doesn't have a retina display. I nearly got the top of the range MBP 15" this time, but in the end decided it was just overkill for my needs. I'm really pleased I got the Macbook -- it's certainly not pointless for me.