I spoke with a guy who has the Shimano Di2. One of the main benefits is that you don’t have to index the derailleur… it automatically does it and always puts you in gear without any BS.

^^^ This.

Cables work fine when you're dealing with nine or ten rear gears, but going up from that to eleven or more gears, indexing becomes a problem, and an electrically-operated derailleur that can hit a gear correctly, quickly, every single time is nice.

For casual riders this probably doesn't matter, since people ride around on badly-tuned derailleurs all day long and just put up with it. Heck, even recreational racers probably don't need it. This is for guys wearing yellow or polka-dot jerseys around France, for whom milliseconds lost to shifting make a real difference.

I'm nowhere near good enough for this to make a difference for me, and I wouldn't want the complexity, which is why my commuter has no gears at all--I was tired of fiddling and wanted something that would never, ever break.

I feel a wired solution would be better, more reliable and more secure, but wireless is the new black.