Boeing uses sheer pins to allow the engines to separate the aircraft in certain extreme circumstances - e.g. rotor seize or a wheels up landing - to avoid more serious damage. For example, the stresses from a high-power rotor seize can potentially fail the wing structure - better to let go of the engine.
Nope not any more.
After the El Al crash the 747 fuse pins were changed to steel and additional attachments were added to all 747s to prevent separation. I believe this is the case with all Boeing now.
I have one of the old fuse pins on my desk as a paper weight.