>Would you please elaborate more
I was inferring that from the discussion I pointed to.
> This cannot be fixed using CSS only because the single post
> page source code is messed up. The only way to fix it would
> be modifying the source code of the template itself.
... where you put considerable effort into getting to the bottom of the problem. I now see that perhaps this precipitated correction of that theme's code, so maybe it's fixed in that case.
Regardless, between themes, and between the different page types (eg: front page vs individual past, with/without various options) there is understandably variation in the html output and thus in the specifics of addressing the elements relative to which one needs to place the tagline.
So, the insertion of tagline using text widget is a trick that has some chance of being sufficient in specific cases, but in advance one doesn't know that it will work, and whether a request to support will be needed to get some minor code anomaly fixed. Ie: when a user's at the point of comparing themes, including ones that lack taglines, they don't know whether activating a tagline is going to be easy, hard or impossible.
It's a lot less robust and fiddle-free than simply having themes consistently output the tagline in a predictable unambiguous way, with a simple option to show/hide, possibly via CSS.
That said, I'm not the one to set priorities around here, for sure. Just commenting on what seem like relatively easy-to-address issues that make adoption easier, users happier, reduces support issues etc etc.