If it works as designed, meaning the FP+ line isn't longer than standby, I don't really see the downside.
I was a FP+ hater in the beginning because they had not worked out the kinks. Now, it's very difficult to argue it's not a better system than before. You know you want to ride x,y,z in advance, so it's not really like, "OMG, why do I have to plan every ride?!?!" This system just guarantees you'll get to ride it at a time of your choosing.
I suppose you could get more Fast Passes with the legacy system, but if they actually enforced the time windows (which they started doing more toward the end), you couldn't hoard them. They also tightened the amount you could have at once at some point.
The new kiosks and cast members helping have been a welcome addition and totally streamline getting more Fast Passes after you use the first few. My only regret is they don't allow multiple park Fast Passes, which I'm not sure why because I don't really see the downside.