I read finished webcomics, yeah. My go-to example is Evan Dahm's Rice Boy and Order of Tales - I started reading Rice Boy when it was about halfway done, but have gone back to re-read it from the start since then. With Order of Tales, I waited until it was finished and then read the whole thing.
One of the great benefits with reading a complete, finished story is that I get to read it at my own pace, rather than at the pace dictated by the creator's updating speed (which is obviously dictated by the spare time they have to create it), AND it's easier to get an overview of what's going on, especially if it's a complex story. It's easier to keep track of stuff like foreshadowing or subtly hinted subplots and stuff like that.
It also means that I won't have to deal with unexpected hiatuses or changes in updating schedules. While such things never make me upset with other webcomics (I'm getting the comic for free; I'm not gonna complain about when or how it gets to me), they do mean a break in the reading flow - and with finished comics, I never have to care about that.
A finished webcomic is a bit like a graphic novel - finished, self-contained formats that you can read at any pace that pleases you. I was reading graphic novels and finished comics in print LONG before I started reading webcomics, so I'm used to the idea of reading something all in one go, and not having a comment field to interact with; it doesn't reduce my enjoyment of the comic. I'm there for the story, not the comments. While fandom can be great, it's never been the point for me - and sometimes, particularly virulent fandoms have actually scared me away from the source material (shipping wars are seriously unsettling at times).
.... but what have you got to lose? Couldn't you read the first chapter or whatever, and decide for yourself if it's interesting enough to continue - you know, the same way you'd do for an ongoing comic?