It seems to me that it would be really useful to have the dependencies of a jar listed in its Manifest, possibly as an optional field. Without this feature, it is near impossible to know what a jar depends on, without finding it on the web somewhere, or using a dependency management tool, or something along those lines. Now that everyone is using the whole artifact ID, group ID, version convention, I would imagine this would greatly simplify the tools needed to setup a project.
I find myself needing to use maven more than I would like to because it's such a hassle to figure out if something needs Apache commons or logging or whatever the case may be, why not just have the jars listed in some Manifest Field?
I think maven is more convenient. Listing dependencies in a manifest is very ambiguous, you don't specify such elements as group, location or repository, even you don't have security on the name of the dependencies.
Describing dependencies in a a manifest is possible, but how do you reference this dependencies? It depends on every developer, and thus can lead to use different names, different jar packaging, etc.
Maven is more complex but also more adecuate, because it defines repositories, groups, artifacts, version and packagins which in the end helps to reduce ambiguity and allows you to rely on a trusty system.
When using maven, a pom copy is stored inside META-INF directory and here you can find dependencies. This is more useful than listing dependencies into manifest.