I am continuing to download to PC and keep a back up. I have unlimited broadband but my provider does throttle speeds if I use a lot and recently due to the many app updates (including some of the larger apps being updated almost every day due to bugs) I've been suffering from that recently. Thus I need to only download updates once (to my PC) to then synch to my three devices. I also presume, because I have the apps backed up on my PC, when I restored my iPhone 6S plus from my iPhone 6 plus back-up, the reason the apps went straight on to the phone was because they were available on the PC with my phone back up containing game scores etc.
(If anyone knows if they don't keep apps on PC will they be immediately available to go on to the phone or will they be re-downloaded from the cloud??)
My own thinking is that if you only have one device, have no problems with data provider slowing your downloads due to heavy use, and the time to wait for all apps to re-download, then you do not need to keep them on your PC. My app library is around 72 Gb but I also have a 2 Tb hard drive on the computer so that amount of storage doesn't worry me - although I do need to go through my apps to see if I really do want to keep them all (downloaded a lot of freebies in the early days which I probably no longer need).
As to other purchases made on devices backing up, so far I've purchased one book via my phone and found it did not automatically transfer to my PC, I had to select transfer purchases to move it into my iTunes library on PC.
Also, the Apple apps that you get free with iPad/iPhone (Numbers, Keynote, Pages) did not show on my PC as an update but did on my phone. I had to go to the app store where they did show as updateable but presumably because they became available free with specific devices they've never shown in app updates but before, of course, I was able to download them to device and transfer back.
Although it is mildly irritating for me to have to wait ages for my PC downloads, I'm now in the habit of downloading to PC and synching to each device and accept that if I am away from home app updates now have to wait until I'm home.