Karune replied a Medivac/Reaper combo forum thread explaning the mechanic behind using Medivac Dropships and Reapers in your Starcraft 2 strategy.
Karune: To give a little more clarification why you would still use Reapers with Medivac Dropships rather than just dropping Marines:
1. Reapers are much faster for raiding. In dropping units (like Marines), there is a 1 second delay between each unit that is dropped. In that 8 seconds, to drop all 8 Marines, you can do a lot of damage with Reapers.
2. Reapers are anti-light units, making them very effective against resource collectors, Zealots, Zerglings, and even a Queen.
3. The Reapers’ mines are extremely effective at taking out buildings for quick hit and run tactics.
4. Reapers can ‘dance’ up and down cliffs to keep your enemies guessing when you’ll attack with little time delay, as opposed to loading and dropping Marines.
I hope that gives a little more insight behind the idea. Nonetheless, the ideas that the community is coming up with is great, and be assured that the Devs do read the feedback.