rMB purchase is probably a luxury gadget for many of us...
Why would we feel guilty about buying it?
I can somewhat understand the potential to feel guilty for an "unnecessary" purchase.
Probably most people posting on Macrumors already have one or more Mac computers, or at least have constant access to a computer at home or work.
And probably the majority of those computers are of similar, if not greater, power than the rMB, so buying an rMB is not necessarily an "upgrade" in the performance department.
So, while the rMB is a great piece of design and engineering, for many users the primary advantage is its lightweight and nice display. Now, this may well be a serious consideration for someone who can benefit from a highly mobile computer.
For others, and I suspect the majority fall into this category, it's an object of desire, and an attractive addition to ones collection of electronic devices. Whether you actually *need* one is highly subjective. I have an rMBP 15, which I carry to work every day in a back pack. Would I like a lighter computer for use on less intensive tasks? Yes, sure - it would be great to carry around to meetings, to lunch, for business trips and on vacation. Could I, hand on heart, say I needed one? No, not by any means. I can carry my rMBP, and deal with the extra weight, and somewhat reduced battery life. It's very much a first-world problem.
Hence, if someone pays considerable money for something which is an unnecessary luxury, one might feel a tad guilty about the indulgence, either because you might otherwise use the money for something more "creative" (e.g. investment, savings, education, gifts to loved ones, charitable donations), or because you're aware that the money you drop on an rMP would be a huge deal to 90% of the world's population.
The difference is that we can justify an rMB as "a tool". Someone who spends $20,000 on a diamond ring, a designer dress or such like, has to find some other "justification" to themselves. Either that, or they just live in a world of privilege without giving much thought to those with less purchasing power.