Good question. The link posted below has the rationale. But I can elaborate a bit... See below:
On Sunday, May 19, 2013 11:12:08 AM UTC+2, Zac Tolley wrote:
Excuse me for being a bit dim, but I'm not totally clear what the story is with Calabash and Frank.
I've seen a couple of messages saying how calabash is a plugin for Frank, or merging it... so trying to understand, are Frank and Calabash-ios becoming one single test framework/tool or will they continue as 2 different things that compete (in a friendly way of course).
They'll continue both, but parts of Calabash will be removed and use the infrastructure provided by the Frank project. So basically you can think of Calabash as an "add-on" or "plugin" to Frank.
I really like the calabash console and the fact that its very easy to build and run tests for actual devices, I also like that I just have a drop in framework folder and which makes the project nice and neat, but I also appreciate the Frank symbiote tool and there is a large installed base of Frank users.
The point here will be that you'll get the benefits of Calabash *and* Frank together in one package. There will be a Calabash console and the ability to run "external UIA" on devices as you do now. There will be symbiote. Here is a sample screenshot of Symbiote running the WordPress app using Frank+Calabash and specifying a Calabash web-view/css selector:

I ask because many iOS devs looking at tools to use for testing and come up against the question... "Should I use Frank or should I use Calabash".
Yes, I understand. Soon it won't matter since we're working on the integration already. But let me give you my opinion if you do need to choose now.
Right now I see three main reasons for picking Calabash, and if none of those apply, my advice would actually be to start with Frank and then add the Calabash plugin if needed (i.e., as it becomes available).
1) You are testing a hybrid app, or need some of the other extended query capabilities that Calabash provides.
2) You need the of gesture recording/playback that Calabash provides.
3) You have an Android app also and prefer to use similar client-apis for testing.
or (4) you're testing a Xamarin app :)
Hope that helps