Earlier quoted context omitted.
This makes no sense to me as there are a load of things good testers do which shouldn't be in instructions and most developers don't think of, otherwise they'd have coded against it. Even silly things like whacking a single button loads really quickly to see what happens. Did you just order 100 tickets? Take down the site? So telling someone to write better instructions seems a bizarre approach.
The purpose of instruction based testing is usually to avoid regression bugs and to make sure requirements are fulfilled. Randomly poking around is also good, this is why you have manually based instruction testing instead of just automating everything, while performing the instructions you usually notice weird things on the side. Neither alternative can replace the other fully.
You just described my job. :)