One reason they omit is that, by default, bookmark text is (was? I hardly bookmark any more) the text of the link. So, if you don't curate your bookmarks carefully, you get a folder full of bookmarks called "Click here!"
Based on the comment I'd expect an "add link to bookmarks" entry in the right click context menu, but I don't remember ever seeing it. It makes sense to use the link text in that case though, else the browser would have to access the underlying webpage to fetch the title. Which shouldn't be a problem, but at one point some web apps were broken and did destructive actions on GET requests, and Google Gears tried to optimize the internet by prefetching webpages... which caused some whoopsies.
But, thankfully, web developers and web technology improved since then.
I think it depends on how you get the bookmark. As far as I can tell, on mobile Firefox (the mobile browser I have easily to hand), you can only bookmark the page you are currently visiting, where the default bookmark title is the title of the page. But, on desktop Firefox, you can also create a bookmark directly from a link, in which case the default bookmark title is the text of the link. This makes some sense to me, since you probably wouldn't want the act of bookmarking a link to fetch the linked page just to find out its title.
