The reason is that symbolic engine does not know what 'rad' or 'm' or any other unit is. Specifically, PTC Mathcad tries to symbolically sin(Pi*rad) and sees 'rad' as undefined symbol and not 1. This is same in Prime 2.0 and 15.0:
When you use :=...= expression, defining 
is the same as definig 
. Similarly, defining 
is the same as definign 
.
The latter is where 15.0 and Prime 2.0 differ. To see advantage of Prime 2.0, consider:
Bottom line: :=...= is _not_ just a convenience for displaying values, but it affects semantics (of symbolic evaluation).