Poking Fiddlesticks generally goes like this:
"Time to poke the support off so I can get my farm"
gets hit by crow, ally frantically moves away from you so that you don't both take triple damage
"Great well I lost 15% of my health but at least now he can't silence me"
Gets one spell off, then gets insta-feared
Spend two full seconds uselessly crawling away from two enemies who are both pounding you
Finish combo while Flashing for your life
Fiddlesticks is down half his health, but so are you and you no longer have Flash
Fiddlesticks heals up off minions, while still throwing the occasional crow your way to prevent you from backing
If Fiddlesticks is at or above level 6, he dives you under tower with his ult, otherwise he just forces you to back far behind your tower where his crows can't reach but you're vulnerable to his jungler
Exhaust: -30% MS, -40% damage reduction for 2.5 seconds, cooldown 210
Terrify: -90% MS, -100% damage (except for DoTs already applied), cannot flash, forced to walk away from Fiddlesticks, any channeled ability you were using is now on cooldown, for 1.25 - 2.25 seconds, cooldown 15-11
Level 1 Terrify is 1.5x the utility and 1.25x the damage reduction of Exhaust, on 7.1% of the cooldown.
Level 5 Terrify is 3x the utility and 2.5x the damage reduction of Exhaust, on 5.2% of the cooldown.
Mega Exhaust should not be a champion ability, and if it is it should be on a long cooldown timer.