Ninja Tabi are a better purchase against Nasus and Ashe due to their dependence on basic attacks and they can't prevent the mitigation. You likely won't be able to escape them with Boots of Swiftness unless you have a dash or blink to prevent them from reapplying their slow.
Boots of Swiftness (and Alacrity) are better for roaming the map, are considerably cheaper than other options, and retain their increased movement speed in-combat (making them preferable over Boots of Mobility on roaming supports to prevent getting caught).
Slows are probably the most common CC in the game. Even excluding Frozen Mallet and Rylai's Crystal Scepter, there will pretty much always be someone on the enemy team capable of slowing you. In addition, +20MS is quite significant for chasing and escaping and roaming.