As Drew mentioned, the cavitation erosion you see on some diesels isn't as common in gas engines although it can happen. The other thing to consider, is that just like in the oil system the cooling system is exposed to pressure spikes and the core plugs are exposed to more than the 14psi system pressure at times. With a good water pump and high RPM you can see 30+ psi in the block and pressure spikes depending on what type of thermostat you run. I really dislike the normal "disk" style thermostat. A rolling sleeve style is both better at controlling temp, but they also don't slam shut and pop open like a typical thermostat will.
A properly sized cup/welch plug shouldn't come out, but in more demanding uses they can fail.
I've never used JB weld for a sealer, but I have used Permatex aviation sealer or something similar (whatever I had handy). I always figured it would be hard to hurt anything with the sealer and it might help.
