It should either be 1. removed, 2. moved to the 9 point tree, or 3. buffed to the level of the deeper trees to warrant picking over swiftness/legendary guardian/etc.
I have never gotten this. The only way this is remotely useful is if all of the following are true:
- the enemy has a frequent slow
- the enemy has the same movement speed as you
- you both possess a slow that slow for the EXACT same amount
If at any point any of these are true, the amount of bonus MS from swiftness is going to be massively outweighed by the difference in slow. In a best case scenario, let's look at Udyr with 525 MS vs Thresh. Normally, Udyr going through the box will wave an amazing 115 MS. With swiftness, he now has a whopping 136.
O wait the enemy is going at 380 MS.
So it doesn't matter. NOW, if you are Ashe and Karma vs Varus and Nami, and you, Nami, have 0 AP but Swiftness, you will be able to escape Ashe with Rank 5 Q even if she has MS quints because of that swiftness mastery.
If you're Karma fighting Nami, no matter whether you have swiftness or not, you will always needs E to escape her Phage. Your slow is not close enough in efficacy for Swiftness to help you escape.
Slows come in a much larger variety than base MS, from 10% all the way to 99%. As such, Swiftness is much more niche. Boots of Swiftness must have that amazing bonus MS because without it, the slow resistance would have to be buffed greatly as the difference in MS does help in times of escape.