I find the web unusable without reader-mode these days, to the point where I host a mirror of a blog I follow just so I can support reader mode on that website.
Reader mode is a godsent that solves the webdesign-issues of custom fonts, colors, scaling, etc. I don't care how people implement their websites, if the content is text-focused, all I care about is to actually read the content. Gone are the days of zooming in and wasting expensive attention on custom implementation of text on background (!). Reader-mode does that, it's amazing. However, I have to either manually enable reader-mode every time I visit the pages (i.e. click extra with *every visit!*, or I have to trust addons. Both of those are not acceptable to me, which is why I need to use Safari for some of the websites I frequent the most (as Safari enables activating reader-mode per default on urls). I would VERY much like to use only one browser.
Please consider this, it shouldn't be very hard to implement and I can't really see how this hasn't been implemented yet. If you aren't using reader mode, you should start doing it.