Explicit channel definition is a requirement when using dynamic destinations (meaning any destination that is added programmatically and not defined in the BlazeDS
services-config.xml, i.e. the destinations created by the remoting-destination tag and the various *-message-destination tags). See
Adobe's documentation for more detail.
The only way you don't have to explicitly define the ChannelSet on the client is if
you are using explicitly defined destinations in services-config.xml (i.e, not dynamic destinations) AND you compile your flex client against that file
your destination is using the application-wide default channel AND you compile your flex client against that file
Even if you weren't using dynamically created destinations it is debatable whether it is a good idea to ever compile your client against services-config.xml, thus coupling your client to your server configuration. It is often desirable to keep your flex client and your server side code as two distinct modules, but compiling against services-config.xml blurs the lines between those modules.
Our recommendation is that it is generally cleaner to keep the client-side configuration of ChannelSets explicitly contained within the client module. An excellent way to do this without having to hard-code the URLs in your client code is to use an ActionScript DI framework such as Spring ActionScript (a Spring Extensions project, formerly known as Prana).
If you choose to go the route of compiling your client against services-config.xml, note that you can at least keep the URL information out of the client code by using ServerConfig.getChannel as described in the referenced BlazeDS documentation.