The title attribute is a standard feature in HTML, and the WP image insertion dialog still supports the input of a title text which is then saved in the database.
When I created my blog, that initial title text was used as img title attribute value, showing up as a tooltip in the browsers. After that, the title support has gradually decayed. At one point, the title input field of the image edit dialog ignored the initial value from the image insertion dialog, so you had to rewrite your text in the edit dialog to get a title attribute. This means you had to use both the insertion and the edit dialogs for every image to get both an image and a tooltip for it. And in v3.9.2, title was completely removed from the edit dialog, so the only way to get a title value for the image is now to hack it into the HTML tab.
What was the reason to phase out the title attrib? And why do it in such a slow and painful way? We still have the title attrib in the insertion dialog, but what are we supposed to do with that info? Is that going to disappear too in the future?
Whatever you do with title attribs, please don't filter them out from the HTML. It is our last way of supplying titles to images. They are standard in HTML and they are doing no harm to those who don't use them. I am really curious what was the reason to put such effort in getting rid of them in WP.