Visual debuggers should be enabled to highlight the current expression
being evaluated rather than just the line it is on. For example, if
the user is stepping through code, the following code will be highlighted
many times:

x = foo() * bar() + baz() / smurp();

yet it will be unclear what method or operation is about to be performed
and thus when to step-over or step-into. In other cases it will be
unclear where an exception was thrown, etc.

Basically, line granularity is insufficient information and is incompatible
with modern visual debuggers.