It's currently difficult to mention other users with DiscussionTools in source editing mode, more difficult than in normal section editing.
Many users' signatures do not display their username exactly as MediaWiki sees it (adding spaces, using nicknames, etc). In normal section editing, this isn't a problem, as I can just copy the username out of their signature and use it in the ping template. However, because I don't see the source of the previous message while replying in DiscussionTools, this is not possible. That means I either have to open the section editing interface, open their user/talk page, or cross my fingers and hope I spell someone's username correctly to successfully ping them.
Visual editing mode has solved this problem with a mention button -- that button should be available in source mode too.