I do this, partly. I mean, I do the early access to upcoming pages, but I also do extra comics for my backers - mainly monthly Q&A-comics and an extra comic strip between chapters. I am also planning to do a special, backers-only oneshot once I hit my 100$-milestone. It would be separate from the main storyline of Grassblades, and self-contained, so it wouldn't be necessary to read it to understand the main story.
I also have a handful of unrelated non-Grassblades oneshots that aren't available anywhere else that I send out to backers of higher tiers.
To answer your specific questions - no, I don't think it's an awful thing to do, even if a creator was to put their entire comic behind a paywall. How and why people do what they do with their comics is up to them, and I don't have a moral right to access those comics. I do however think that if a creator puts their comic behind a paywall, they have to be reliable about updating on time. A free webcomic going on hiatus or just stopping for no reason is just a disappointment you have to live with - but as soon as you take people's money, you had better deliver, or you need to refund them. Even if it's just a buck a month, you still make a promise when you take someone's money, so it's on you to fulfill it.
I don't think there's any one way of doing the updates, either - complete downloads work well when it's oneshots, or if the creator can reliably complete a chapter/issue a month (as most Patreons are structured on monthly payments), but page by page uploads should work too. The only real issue there is that Patreon doesn't have a searchable archive of posts, so a new reader would have an awful lot of scrolling to do to get to the start of the comic - it's not like a webcomic site.