I don't understand why someone at Dropbox's size and scale would go after C++ on mobile and blaze their own trail, when its probably more pragmatic to go native.

With that being said, I worked for a firm that extended the life of old ERP systems and we had to do a few mobile apps, we chose Xamarin because we were a shop of 7ish devs that had many projects to maintain and the cost of code sharing and familiarity with C# were our driving factors. It was a trade off most definitely but it was a pragmatic choice as well.

If you are capable of hiring more developers who know Java or Obj-C and can allow them to do only mobile development then its worth it to go native.

But, if you are a small company and know that Java,C# and Obj-C, C# developers are a little bit hard to find (depending on your developer market) then its probably more cost effective to go Xamarin or any other cross-platform code sharing model.

In the end its all about pragmatism.