User blog comment:Alartan/Swiftness or Mobility boots?/@comment-5266525-20150201191955

Alternatively, just get the Alacrity Enchantment. It works wonders for this exact reason, giving you as much movement speed as a champion with tier 2 non-Alacrity boots (which is not uncommon) so you're not handicapped as soon as the fighting starts.

Regardless, Mobility boots work best on champions expected to initiate because it adds reliability to their engages. Being able to open a teamfight or gank by just walking up to someone and hacking away without spending any other resources is well worth the loss of 40 movement speed. You might as well say it's 'saving a 10s CD ability vs getting a small bit of extra in-combat movement speed'.