[plt-scheme] PLT's debugger philosophy

Last week I watched a movie that demonstrated how the MIT Scheme
debugger functions. Apparently when an error occurs, you can start the
debugger at which point you've got access to that stack frame at the
location of the error at which point you can evaluate expressions and
muck about with the environment if you desire. I haven't explored it
any further than watching.

The thing that struck me is that it is different the how the debugger
works in DrScheme.

Do these two approaches reflect two Scheme debugger philosophies?

What is the philosophy behind PLT's debugger?