A simple click on a button in the bookmarks bar (BB) opens the site in the current tab. This is useful for javascript bookmarklet (my favorite being displaying the [TITLE] in the page)
But when I open a real website, I almost always want to open it in an active new tab. Currently, I use Ctrl+Shift+Click to achieve this result (or Ctrl-T + click). Having a checkbox to allow the link in the bookmark to open in a new active (not background) tab may be useful.

I want this because I gain productivity. Links in BB are either bookmarklet or sites I need for quick overview (cheatsheet, transportation). Usually, their url is not distinctive enough to use Ctrl-T and the search from address bar, because there are too many results to filter out.

I hope I made my point clear