User blog comment:Alartan/Swiftness or Mobility boots?/@comment-25003900-20150203162951

Frankly the boots of Swiftness are bought so infrequently because the extra 15 movement speed and reduction in slows by 25% isn't that useful when you consider that you could go Merc treads and have 35% reduction in all CC including slows as well as 25MR, or Tabis have armor and a reduction in the damage of basic attacks. If you would go swiftness, just about any other boot choice is more optimal than swiftness because it's literally just speed.

The only champions I'd take boots of swiftness on would be Hecarim as he's the only one that will actually make viable use out of the extra chunk of speed because that bit of speed grants him damage, and I'd also consider Olaf in some cases. For everyone else there are better options depending on your role in the team.

Mobility boots are an entirely different story. As you stated above they're there largely for jungle dominance on junglers with reliable gap closers. Rengar, Lee, Warwick, Xin, Jarvan can make decent use of them.

Now I also agree with Shaw below in that they can be useful for extra engage effect, but I'll differ and say that it's typically from a flank, while Alacrity can solve a lot of the problem, if you're expected to initiate you'd likely be better off with Ninja Tabis or Merc Treads because of their extra benefits. In many cases a tank or bruiser will initiate and they make better use of boots with other stats.