Often times they are repairable, but the team will decide to swap it out and repair it at the shop where they can take the time to make sure it is done properly and decked or line honed to be perfect instead of "Close Enough"
And if you think a crank can be swapped between rounds, you have never degreed a cam in a Fuel Engine. You don't just drop it in and go. That's why the short blocks are built in the shop.
You also mentioned a cam swap, (same issue degreeing it in) and if a lifter fails they often get stuck in the block. Sometimes a slide hammer will get them out and the lifter bore is OK, sometimes not. So they swap the short block and fix it at the shop. That drastically reduces the chance of a catastrophic failure if a new lifter is put in a galled bore and run.
And I know you were kidding, but I have seen JB weld used in a pinch. Sometimes it even worked, but I wouldn't want to see it become normal.
I do think that if a team can fix it with a band aid that may or may not hold a better option is to swap it out and fix the damaged one properly at the shop. Not only is it safer, but the chances of causing an oil down are much less as well. If you think the teams don't care about blowing up engines, go back and look at Jim-Os interview last week in Dallas.
Alan