What is the purpose of exhausting Galio when he ults?

In this game, Galio is repeatedly exhausted everytime he uses his ultimate ability. However, Galio's ultimate roots him to one place. What's the purpose of exhausting Galio when he uses his ult?

http://www.own3d.tv/clgame#/watch/529435

exhausting also reduces attack and ability damage:

Slows movement speed by 40% and reduces (auto/basic) attack damage by 70% and ability/item damage by 35%.

and if you have improved from talents it also reduces resists

Improves Exhaust to also reduce the target's armor and magic resistance by 10 for the duration.

Source

The reason regarding 35% damage reduction might be the after-the-fact justification for why people do this, but honestly it is more likely that it is a gut reaction to a fight-changing ability like Galio's Ultimate. They know that they want to do SOMETHING to stop him, and perhaps exhaust is all they have, so they throw it down. I cannot think of a situation where exhausting an ulting Galio would be more valuable than saving it for a more dangerous enemy champion.

Just my two cents.