It's not about epistemic closure or creating an echo chamber, it's about improving the signal to noise ratio. Imagine you have 30 minutes a day to goof off here. Would you rather see all the most interesting threads and posts on the subject, or a random sample of people posting "qft . this ya m$ sux dix" crap? Suppressing posts by specific users is admittedly a crude way to do it, and even bad posters make good posts (and vice versa), but what's the problem in trying to get more bang for your buck by concentrating your time on posts who actually contribute?
I don't use the ignore list since I've been a mod because, you know, you have to be able to moderate bad users, but because before I was a moderator I used it extensively and wrote an extension for my browser to actually remove posts by ignored users and also to be able to remove threads I wasn't interested in. A 100-post page had about 15 posts and a page of threads had maybe 8-10 threads, but it also meant that the average quality of post was significantly higher and I got to contribute to stuff I was really interested in.