1. It is a single shield to plug into your Arduino without having to wire up discrete components.
2. It comes with a library.
3. The Adafruit shield in particular allows many shields to be stacked and addressed individually.
4. It is cheaper for the manufacturer to buy components in bulk so the final cost is less than you can buy the parts for.
1. I understand the value in plug-n-play with the UNO or MEGA. If you are driving that shield with and UNO or MEGA, then using that form factor for prototyping would be beneficial. However, in trying to minimize the space requirement, I'd rather go the route of using discrete components (if I had the electronic/programing know-how).
2. The only point I would argue here is if I were using an Arduino in any form with 3 Mosfets, I wouldn't have to use any library. I do think it's pretty cool that the Adafruit people wrote a solid library base for their shield though!
3. Very handy if I had space and needed to control various motors in this fashion. Maybe this is where the value tipping point exists. I would assume that it would be easier to use multiple shields than extraneous components. I dunno....I still missing a ton of experience in this area.
4. This surprises me. The Adafruit Motor Shield "kit" is just the shield itself, priced at 19.99 (no motors included). If I'm just powering 3 motors for example, I would need 3 Mosfets (I think). Are mosfets typicall expensive? My thought was that I could buy a smaller Arduino (Micro), 3 Mosfets and a proper power supply and that should cover it. Where is the price savings?