Yes, agree. I didn't see the optimist module when started to work in this, it seems a really good idea, at first it was add android and iOS, both platforms, that's why I'd choose to add all possible platforms.

We have options to pick, we can add the arguments like the platforms, android, iOS, blackberry, wp8 and so. But this allows to the user add platforms that are not allowed to use in their environment, let's say:

Mac Os dev env for Android, iOS
If the user types: createmobilespec android ios wp8 windows8
the last two of them will cause problems under that dev env.
the same thing if it were for windows 7 or XP, it's not possible add a platform like iOS, it would create an exception during installation that stops the execution.

So, it would better try to prevent this errors.
We can add logic for when user try to use a platform that is not allowed to warn him and stop the process immediately without create the project and nothing, I mean not waiting until the exception or error is caught.
Or
We can make it interactively, add a menu when the time to add platforms, after the project creation, allowing to add just the supported platform for the current environment.
Menus like the following ones:

Mac OS:
Select platform:
A) Android
B) Blackberry
I) iOS

Windows 8:
Select platform:
A) Android
B) Blackberry
WP) Windows Phone 8
W) Windows 8

Windows 7, XP, Linux:
Select platform:
A) Android
B) Blackberry