Stomp is a simple and public protocol: http://stomp.codehaus.org/Protocol
And it doesn't specify any ObjectMessage on it. It's all about sending and receiving messages. (i.e. ByteArrays).
Also, Stomp is multi-language, so you can't assume Java.
The rest interface is something different, that's being created by Bill Burke, so he could be more creative and add more options for users.