I think the idea has been mentioned many years ago - maybe by Jonathan Lewis in Cost Based Oracle Fundamentals, or by someone else somewhere else...
The A-Rows information displayed by different trace utilities (rowsource statistics in dbms_xplan.display_cursor and sqlmonitor) shows the number of output rows returned by a step in the execution plan: but there are situations in which it would be also of interest to know the number of input rows for a given step. A fitting example is Explain plan control - hash join running long: a hash join returns just a few rows (shown by sqlmonitor as A-Rows), but the intermediate result contains billions of rows that are excluded by a filter.
So it would be a helpful to gather this information with the rowsource instrumentation code (if this is not already done) and display it in the corresponding plans.
16.04.2015: Randolf Geist provided are more complete explanantion of the problem (and possible solutions) in his blog: Oracle related stuff: Combined ACCESS And FILTER Predicates - Excessive Throw-Away. And I also think that AE-rows (Actually Evaluated Rows) would be a good name.