I see people buy those on supports. But I have NEVER been a fan of those.
Maybe I'm wrong. But this is how I see it...
Swift boots give extra speed + slow reduce at ALL times.
Mobility boots are useless in combat.
Sure if you roam alot Mob boots is useful, but I feel it is hard to predict sometimes how much company your adc will need. You never know...
Alternative I love the Merc boots for tenacity.
Support can get CDR cap at 45% very easily already. Both coin and spellthief gives 10%, now most people build redenption. Solari is also good but more situational. Every support item got like 10% cdr.
If you are tank you can get frozen heart 20% cdr (quite good in most situations) even if you don't get it there are other alternatives.
I never buy cdr boots. I just don't see how they are useful when you can get plenty of cdr from other items.
Mobility boots can pass, but cdr boots just sux... yet people buy them a lot. Why? xd