Yasuo with exhaust
Exhaust
Exhausts target enemy champion, reducing their Movement Speed and Attack Speed by 30%, their Armor and Magic Resist by 10, and their damage dealt by 40% for 2.5 seconds.
Ignite
Ignites an enemy champion, dealing 50 + (20 × level) true damage over 5 seconds, applying Grievous Wounds icon grievous wounds and revealing them for the duration. The sight will not reveal stealth champions.
Exhaust vs Ignite
Both point and click summoner spells. Ignite deals true damage over a period of time and reduces healing effects on Champions (50% reduction). Exhaust lowers the enemy's damage output, lowers their attack speed (further lowering their damage) and reduces their defenses. Both spells have a 210 second cooldown.
Importance of an Early Kill
An early kill is massive. Not only you deny the opponent farm, but you also get to farm for free, and extra gold. It's amazing to get first blood, however:
Good Players play safe. Ignite is only good if the enemy is going to be really low
Exhaust protects you and others from ganks. You can reduce the damage taken by one while you kill the other, or you just stop the engage
Yasuo is squishy what good is ignite if you're not going to take advantage of it? Exhaust allows you to not only do more damage (late game) but also survive longer, which in turn makes your damage be even greater
In Sum While ignite is a good spell to snowball and to stop those pesky healers (Soraka, Sona, Swain, Fiddlesticks, anyone with lifesteal), Exhaust is able to save lives and do damage in the process, while also being extremely effective in teamfights due to not only protecting yourself but also others in your team. It will also reduce healing effects on certain Champions such as Fiddlesticks (His heal is a percentage of the damage dealt, he does less damage, he heals less)
It's mostly a trade off :
Early Strenght, Single Target Late Healing Reduction
against
Early Safeness, Multi Protection Late
In my opinion, I would not take exhaust on Yasuo. The advantages of taking exhaust are that if you are up against an assassin then you are able to negate a portion of their damage. It also slows so it can help with chasing, running, peeling, or help with aiming your tornado. The season 5 meta as it stands right now is moving away from the bursty assassins to bruisers. The kill pressure from ignite outweighs the benefits of exhaust. As Yasuo you will not need exhaust for the slow as you do have pretty good chase potential and outplay potential if you are ganked. The damage reduction would help as you can be fairly squishy.
This is what I think about taking exhaust on Yasuo. I hope that this helps.