Cache your own components using The Flash Player Cache-Collection of common programming errors

if you create your own code library in a swf external to the main swf, this will be cached by the browser for all requests to the same host, so multiple swf files using the same code library on the same host will benefit from this. In the case of signed RSLs, effectively, Adobe is allowing their common code base to be shared across domains, for which a very high level of trust must be assumed. I can’t see much reason to require this for your own code, and would view any leniency on Adobe’s policy as a security risk.

edit: With the right crossdomain policies in place, there’s no reason why you could not have a code library shared across multiple domains.