It only adds an 8% bonus to your bonus movement speed.
Look at it this way, teir 2 boots only gives 45 movement speed, meaning that you're only getting 3.6 bonus movement speed. Of course, 90% of the champions in the game don't even care for movement speed, except champions like Hecarm, Rammus, or something like a roaming support. The only other way to use this bonus would to take ghost, which again is VERY champion specific.
But at this rate, is getting another 12-15 movement speed max worth taking over something like transcendence? This rune is superior in every way, 10% cooldown reduction + bonus adaptive damage for capping over. Why would you not take this over Celerity?