You don't need a G+ or any kind of Google account to do other non-Google things, say shop at Amazon or watch a Netflix video. This is the right way to compare. The major difference is that we're not a one-trick pony, we have dozens of services that people want. For a more comparable example, several mostly-unrelated Microsoft online products (Office365, SkyDrive, Xbox, Skype, Outlook.com, etc.) are all under the umbrella of a Microsoft Account. Which also makes a lot of sense, for example I suppose you can buy a unified cloud storage quota that can be used by both Office365 and SkyDrive, among other things. OTOH, because I have a MS account, I also have a (send-only at least) Outlook.com account. But I have no problem with that, I just don't use it.
Certainly. But one difference with e.g. Skydrive is that everytime I upload a photo to what used to be Picasa (now Google+ Photos), it offers to share on Google+ and I have to skip that step.
You don't. You can just downgrade and delete your G+ Profile.
You can very well disable your Google+ Profile and keep any other services. The only extra work needed is in case you have a Youtube channel. In this case you just have to unlink your Youtube and Google+ Accounts first.