Why use RTAI for real-time applications? 
Real-time application need
- efficient executing code
- deterministic behaviour
Actually development for real-time applications is mainly made in embedded systems in the language C.
More and more is done in C++ and on PC plattforms. With D there is an alternative to C++. But there are non-deterministic features in D (like in C++ also)
- dynamic memory allocation
- dynamic casts
- garbage collection
- exception handling
In a real-time environment not all has to be deterministic. So it is in the hand of the developer not to use these features in the real-time task.