Elementalist's passive speed buffs do not stack properly.
Yes the Windborne Dagger trait does not add any speed at all right now. It is also pointed out on the wiki http://wiki.guildwars2.com/wiki/Windborne_Dagger
I hope this gets fixed soon. Its a trait that I would really like to use.
I posted a more in depth analysis of the issue a few days ago, but in short ABSOLUTELY NO MOVEMENT SPEED INCREASES STACK. The fastest any character can go is with the swiftness boon and all characters with the swiftness boon move the same speed (disregarding the special equipment that “buffs” swiftness).
One with Air is working up to the 25%, but it has strange activation and deactivation issues. Look for my post if you really want to know all the gory details.