If users consistently make a mistake that is jarring to their experience, it reduces their willingness to interact with the site. IMO, it's too difficult to get to all posts for a certain tag already, but I can see forcing it to take two clicks as a reasonable way to promote a more comprehensive view of the site. However, that seems like it would also have the effect of reinforcing the popularity of highly-used tags and the unpopularity of less-used tags. If someone sees, say, a post tagged #cephalove (my pet tag for cephalopod posts) and it takes them two clicks + a detour through unsubscribing to get to an all-posts page and see how many other posts they might have missed with that tag, they're less likely to check it -- and less likely to click, read, and submit posts with that tag.