Most of the areas where I hammer-mined recently were completely riddled with about 10x10 (at the widest) bomb tunnels, to the point where I just gave up mining entirely for the last few weeks because I couldn’t find any areas that didn’t look like Swiss cheese.
But on the other hand, I have always refused to use bombs for mining, even when apparently everyone was using nothing else, seeing as they were, in my view anyway, intended as combat tools and not mining tools, so I don’t have first hand experience on their usage. My only experience with them is what I’ve read on the forums here, as well as running into the giant swiss cheese caves that make it impossible to effectively mine with a hammer. And from the post I linked, as well as others similar in nature, people were getting a lot more resources with their bombs than I was with my hammers. 
I’m probably in the small percentage that actually finds it fun to spend hours underground slowly chipping away at rock and making perfectly square and evenly spaced branch mines.
Edit: Here’s another example, posted just now on another topic like this one:
500 diamonds would take me about a week and several stacks of hammers. 