What they said. Only real reason to change is for "status" (having an all-Ultegra bike).
I ride a triple, and got a 105 FD to replace the stock TIagra unit (which was fine at first but got worse with wear). In any standard Shimano triple the shift from small to middle chainring (30 to 42) will always be somewhat rougher due to the relatively large size difference with this shift (12 tooth = 40% difference, vs 10 tooth= 23% diff shifting 42T middle to 52T big ring).
If you are running a well-adjusted FD with a good non-binding cable, I doubt you would honestly feel a difference in shift quality after going through the trouble & expense for this "upgrade". Personally- I would not turn down a freebie Ultegra triple FD, but I would not spend a nickel to change out from a 105.