Wasn't there a discussion of this on CHIRP or somewhere recently?. I seem to recall the outcome was that (in the uk) it was in fact in breach of the conditions set out in a companies AOC and therefore illegal.
The AOC is granted on the basis that a proven and reliable system of fuel planning is in place for each flight.That may be a complex manual PLOG or more comonly a CFP (sword etc).You can't just ignore the data suplied as that data has been judged to be integral to the legal operation of every flight. In flight replanning / decision point procedures etc are a different thing and must be described in your companies approved manuals (also a requirement of the AOC)