To rewire, or not to rewire
My house was built in 1940 and we are gradually rewiring the whole house.  Here are the reasons why:


aluminum wires: most of the original wiring was aluminum which is not as safe as copper.  A particular problem is that there are many devices (outlets, lights, etc) that are rated for copper but connected to aluminum wires.  This is dangerous.
equipment ground: most outlets are not grounded while many electronics require a ground (i.e. three pronged plugs).  New circuits are all grounded.
buried junction boxes: in doing other renovations we discovered junction boxes (boxes in which wires are connected) buried under plaster or otherwise inaccessible.  Maybe 80 years ago that was OK but it is not up to code today.
other safety concerns: We've noticed unprotected wires, NM cables running outside (no conduit), etc.  
high draw devices: we are installing air conditioners in each of the rooms and these require dedicated circuits.  I wouldn't have used hair dryer in the bathroom with the old wiring (safety concern).  


When determining the order of rewiring, we have two criteria:


address the most urgent safety issues first: having a usable, GFCI outlet in the bathroom, for example.
address the wiring that can be done together: we have our kitchen ceiling down at the moment so we've used the opportunity to run some wiring for second floor outlets.  


You might want to have an inspection / evaluation completed and then assess what parts of the system should be rewired and in what order.