Usually you don't want to buy boots of swiftness on any champion except on Singed or Hecarim but even for those two swiftness is very situational and it's usually better to buy Ninja Tabi or something like this.
Now another thing you should think about is: Can my champion really benefit from the stats on Zephyr? Of course Tenacity, CDR and Movementspeed are good on every champ but Attackspeed/AD isn't that useful on all bruisers.
But let's see where you can buy Zephir for extra movementspeed, Tenacity: 2 Champions that come to my mind that work actually pretty well with Zephyr are Udyr and Irelia. Both are Rather tanky, benefit from Attackspeed, AD, CDR and Movementspeed. But boots of swiftenss? The main problem with these boots are not that they are "bad" but that there are other boots that are far superior to them. If you take ninja Tabi for example you will get a good amount of base armor which is very strong early in the game and 10% damage reduction from basic attacks. This means you are negating a huge amount of damage coming from AD Based champions.
Recently riot also removed tenacity from the tank jungle item (Spirit of the ancient golem). This basically means that now there are only 2 Items on summoners rift that provide tenacity and only one of those two Items is viable on a lot of champions (This somehow sounds familiar to season 1 eh?). Unfortunately tenacity is a really important stat if the enemy has Hard-CC with a duration of 2 seconds or longer. So here a little overview of when to buy which boots. Bold Boots are a good option when buying a Zephyr on bruisers.
- Boots of swiftness: Usually you should avoid buying these boots except you are super fed and you want to run around like crazy during combat (which isn't that benefitial most of the time)
- Boots of Mobility: Take this if you plan to roam a lot or on support champions in early game
- Sorcerers Shoes: Bought on most champions that deal Magical Damage. Especially effective early if they have high base damage/low scaling
- Mercuries threads: Buy these if you are a more Bruiser-Like champion and if the enemy has more than 2 seconds of hard CC
- Ninja Tabi: Buy these if you are facing a lot of AD champions or if the enemy ADC is fed.
- Ionian boots of lucidity: Sometimes a good choice on Support champions but you can usually reach 40%CDR without these boots.
- Berserkers Greaves: Take these on ADCs or when playing a glasscannon character that scales well with AS.
The thing with BoS and why they aren't that viable: They reduce the effectiveness and not the duration of a slow. If you are slowed for 40% you will lose 10% on that slow but you are still slowed by 30%. This is still a huge slow when playing the game.
A correct boot choice is important!
Don't be this Riven