Some people want a random subset of news for entertainment. G+ is great for that. Here's a great big pile of recent stuff from everyone, a circle, or a community.
On the other hand, lets say I want to verify I read every story from certain sources (perhaps the BBC?) exactly, precisely once. I don't see a way in the G+ UI to "mark as read" or "only display unread" or whatever. Might exist or might be a weird way to hack it in, but its not intuitive.
For some reason option #1 is semi-popular among news readers, social networking, and old fashioned or online broadcasting, but option #2 is nearly universal among email clients and podcast clients. You'd never use an email client with a UI of "here's a random collection of some recent emails" with no way to tag them as read or deleted. I'd LOL at the idea of a podcast client with "here's a random cast that you may or may not have already heard"
Another horrible UI fail is time based. G+ can only do one linear "newest to oldest" sort. Even the dumbest RSS feed readers have some kind of tag/save/star feature. So you can skim the whole feed, star/save/tag good stuff for later detailed reading. Then once there is no "unread" left, start reading the star/save/tag stories in detail, perhaps much later. This "workflow" is impossible in G+ as near as I can tell.
Finally no one using the "RSS" workflow uses G+, so you get the chicken and the egg thing, where there's no demand for even the most primitive of workflow management in G+ because anyone wanting anything like that uses RSS. So the BBC is a bad example in that their social networking people are probably required to use both, but I suspect that 99% of my RSS feeds are not available in G+ because they self segregate by workflow. You'll need a bigger shock to the system than cancelling goog reader. I switched to newsblur once the crushing demand let up a bit. I'm not going to abandon everything I currently enjoy to watch cat videos on G+.
In addition to VLM's excellent points about the horrible Google+ UI, the feed system is open while Google+ is proprietary and closed. Why should the BBC subsidize Google's business? Why should all of us miss out on innovation which can't happen before Facebook/Twitter adds a feature and Google copies it? Reader might have been the most popular client but there are a ton of tools and services which do interesting things using feeds, none of which are possible in a walled garden.
