We had a FP+ for the Frozen sing-a-long and thought it was worthwhile, since the standby line was quite long. We just showed up 15 min before and got decent seats.
DW and DD did get one for the Disney Junior show but it really wasn't necessary, it would just mean showing up 15-20 minutes earlier in the standby line. She also said the FP+ seating area was not that good.