Firstly there's reproduction size. Yes you can get good results on screen with an iPhone and properly lit photo, it wont look good printed in a glossy magazine, or on a 10 foot advert! I see this time and time again when someone produces an attractive image from an otherwise maligned camera such as a phone camera and uses it to argue that more expensive cameras are redundant, and the image in question is 600 pixels by 400 pixels!

There are other advantages to medium format other than image resolution, that is superior lenses (even the most expensive 35mm lenses costing over $2000 are comparatively mass produced and inferior to the best medium and large format lenses), faster sync speed due to leaf shutters, and better micro contrast on account of the format size.

Another issue is repeatability and reliability. You might be able to get good results with cheaper gear and that's great. But it might also be a lot more work and less reliable, making the expensive gear a better option for a professional. I shoot with a 1D and 1Ds, not because they take better quality images, but because they are more reliable, and have features such as simultaneously recording to two memory cards in case one fails.

Finally pro photographers aren't stupid or wasteful (though it often seems that way!) if they could generally get the same results with an iPhone then most simply wouldn't buy a Hasselblad.

iPhones aside you make a good point regarding full frame DSLRs which are genuinely starting to tread on the toes of medium format, with talk of 30 megapixels plus in the next generation. However the genuine advantages of format size in terms of sharpness and micro contrast (note that I don't consider shallow depth of field to be an advantage of MF due to lack of extreme fast lenses) will always hold out, as will the lens and accessory support.