We could consider a contextRelative="true" flag that would apply only if action is provided at all and if it starts with "/". For example see RedirectView and the Spring UrlTag. The servlet mapping is a bit more tricky and not always possible to determine the actual mapping in containers so I'm less keen on that. If you want to provide any code please consider adding the code via https://github.com/SpringSource/spring-framework-issues.
Having two attributes such as contextRelative and serlvetMappingRelative on every form tag doesn't seem like too much of an improvement. We may be able to decide whether to prepend the context path (and the servlet mapping) transparently recognizing that existing actions work either because they contain the context path already (e.g. with the help of <c:url var=".." value=".." />) or because they're relative paths.
We'll revisit in 3.2 together with the related request #10449 and also in light of work done in 3.1 to build context-relative and serlvet-mapping relative URLs (#13465, #12675).