I guess great minds really do think alike! haha. I've stood in my orchard many many times and considered that. And every time I plant a new tree, I seriously consider just putting in between 2 other trees in the same row. But my problem is that 9' seems a little less than what I'd like. One reason is exactly what you said- I have allowed my peaches, especially, to spread out so far that even though the trunks are 18 feet apart, their limbs are almost touching. SO several of my trees have sort of grown into their space, even though it would have been easy to keep them apart at less distance.
The other issue is mowing. As it is, Its pretty easy for my to get my tractor w/mower between all my trees. At 9 foot spacing I wouldn't be able to get between them with my tractor.
Anyway, its a logical idea and I may well end up doing it one of these days. I actually have a little more than a full acre of land that joins my orchard and would be a logical place to put more trees. That being said, I've always had intentions to try and keep that as an open field so in some ways I feel like I'm running out of space. I guess at some point I'll have to decide to go to 9 foot spacing or give up my field. Knowing my passion for fruit trees, eventually it will be BOTH!!!!! haha