Originally Posted by
mtseymour
I use 11sp mechanical (Sram Red) and electronic shifters (Ultegra Di2), and would not upgrade to electronic shifting for the rear only. Compared to a 11sp mechanical drivetrain, eTap of Di2 only has a slight advantage when it comes to rear shifting. It's slightly faster and will shift flawlessly with minimal maintenance. However, electronic deraileurs have a big advantage when shifting in the front. They're much faster and can handle a large gear gap (eg. from 50 to 34T) without dropping a chain. It's so nice to easily drop into a smaller chainring when the road quickly steepens without having to ease up and wait for the downshift.
If you really want a triple crankset, it's possible to modify the XTR Di2 for this purpose.
I agree that in terms of shifting performance, an electronic RD isn't that much different than a mechanical RD. However, if you run a non-stock cassette than the electronic RD will cope with it far better than a mechanical RD. Also, having an electronic RD gives the option of having a gear display, which is very handy on a tandem. Finally, having an electronic RD gives the option of having shift buttons in multiple locations, which is something that suits me well.