Yes, exactly on the idea. "The weakest spot" is distinctly NOT the hunk of solid rock cooled at the top, more often than you clearly think.

It doesn't have to have a huge effect to never the less completely change the nature of an eruption. Just look at Mt St Helens. The top didn't even have to pop. The magma just lubricated the side of the mountain enough until the whole damn side slid away, allowing the eruption to occur.

They don't often redirect every eruption, becuse many have NO other weak paths, so the magma pushes, and it either pops the rock plug or melts it. Not that many eruptions start fast enough to catastrophically pop the plug, and not all eruptions are remotely equal.

My point was NOT that the plugs make a HUGE difference, but that they make A difference.