Pros:

incredibly high burst through his "E" passive
great ability to assist junglers by puting a lantern in a place where the enemy cant see it and then have the jungler fly in.
if matched with or your resist through there auras and your passives will make you very survivable
good chase and escape through his "Q" and "E" respectably

Cons:

Short auto attack range
No Harass
No AD scaling abilities that complement your autos

You will get some hate for taking Thresh as ADC in the current meta but you may be able to pull an bad counter pick from the opponent.