I don't think it's stupid at all. If you've never seen a motor actually come apart, it's hard to imagine how much damage can be done.

To answer the question, when you think "blow it up", think about parts that were once on the inside ending up on the outside. When this happens, the flying parts aren't terribly particular about what path they take. More often than not, they go right through the block, rendering it unsuitable for future use. Think big $$$.

You can do severe damage to a motor without actually blowing it up. Shattered pistons and bent rods can leave the motor looking pretty good to the naked eye, but still unusable. Melted pistons and bent valves also happen if you're not careful.