The worst part for me is that the XPS13 only allows a MAXIMUM of 4GB of RAM. This is ridiculous for a dev box, especially when I want to run a VM or two.
I agree, running a VM leaves you at the redline. Running 2? Good luck.
I do run with a single Ubutnu VM nearly 100% of the time, but I skimp on it and only allocate 384MB usually. Sometimes I'll bump it up depending on what I'm doing in it.
All that said, I do wish that I could have 8 GB in my Air. But, with the extremely fast SSD, it's not essential. I use this machine as my daily driver (for mostly web-based Python, PHP and Go development) and love it. It's honestly my favorite development machine ever. I've never enjoyed developing on-the-go more, and at home plugged into a Cinema display it's a real joy to use.