I'm sure your friend doesn't need a car either if all he does is sit at a desk in an office all day, but I bet he has one to get to work.

Depends...if you can make the money back in a reasonable amount of time then you ate good...or are you okay with not making it all back but get an amount of joy owning better equipment.... Either way it's what works for you. I think you can easily get decent results from less expensive equipment.

Depends if you look at equipment as an investment or a cost.

Personally, I like investing in good glass since it will always be useable.

If you're only concerned with optimizing your net revenue, then get the cheapest camera that will produce the quality you require, augment it with good quality used glass you can find and use techniques that don't require expensive accessories.  I mean, why use an expensive tripod when a box or a block of wood will do just as well, if not nearly so conveniently?  But of course your time and effort might be worth something, and gear that reduces the effort or increases the likelihood of success (or both) could justify itself.  I bought my D800 in part to get better low light performance on the rare occasion I need it and in part for features like Virtual Horizon that make it easy to get a shot right in camera.

In any event, don't confuse the income from individual sales with the value of your product.  Coca Cola's revenue per customer sale is pretty low, yet they do very well thanks to the volume of sales they make.  And so it is or at least was with microstock; small amounts of money repeated hundreds and then thousands of times add up to something substantial.  This is a volume business; it's the total that matters most.

A final thought that applies to me if not to you: I shoot because I enjoy shooting.  Making money supports my habit, and lets me justify buying better and more sophisticated gear.  The customers get the benefit of that better gear, and so do I.  I'd want that better equipment even if I didn't have money coming in from photography.  Might not spend the money, so I'm glad I can justify it more easily.  But as with many things, your mileage may differ.

'A final thought that applies to me if not to you: I shoot because I enjoy shooting.  Making money supports my habit, and lets me justify buying better and more sophisticated gear.  The customers get the benefit of that better gear, and so do I.  I'd want that better equipment even if I didn't have money coming in from photography.  Might not spend the money, so I'm glad I can justify it more easily.  But as with many things, your mileage may differ.'

This totally applies to me! When I used to play tennis I would spend a few hundred dollars on a high end racket because it felt better and I enjoyed the equipment- did it make me better - probably not but it didn't hurt me either...

At least you can use a camera to the limits of its performance - cant say that about going to the shops in a Porsche!

you don't need a D800 or a 1D or any other high end camera, a crop sensor is enough, I have a D90 and I don't think it is pulling me back in any aspect (ok only iso after 400), of course I would prefer a full frame camera but it isn't a requirement to make money in microstock or photography in general

The car analogy is great. I agree that if you like to own nice things, then why not? I own a D800 now, but confess I often pull out the D700 for client shoots as the files sizes are more realistic for their final needs, and that camera is still set up just how I like it (can you believe they switched the + and - buttons on the 2 bodies, so annoying! and the wheels are set up like Canon so I keep spinning the wrong way, making every shot take twice as long - yes, I know somewhere in the menu I can switch it back to the "nikon" way) . I try to always shoot a few images on every client job that can be used for stock, but at this stage I don't really care which camera it's on, as in general I don't get paid more at the different sites for larger files. I still have files selling from my D80 body, with hobby lens.

I mostly bought my d 600 for display reasons.

I would have done fine in microstock with my old d 200

okay, I will save for the 75mp- I MUST HAVE THAT CAMERA! 

I have a D800 and I'm not making any money in microstock.....

no only the rolls royce but I just got the test results. I am images retaken under the exact same conditions were accepted!  So companies do look at what you are shooting with which sucks!