I'm not able to verify this ATM, but does this mean libraries with names ending in -jure are strictly disallowed? Granted there's probably plenty of mylibjure sort of names out there now, but this change would be quite bold.
You can work on projects with names like that, you just can't use the "new" task to generate them. The official workaround is to generate a skeleton with a different name and rename it, or to use Leiningen 1.1.0. Awkward, but hey--it's an edge case. =)
If you use ant and it doesn't drive you crazy, it may be that your project just has very simple needs. If you read through the tutorial (http://github.com/technomancy/leiningen/blob/master/TUTORIAL...) and don't find anything in there you need then that's great; you should consider yourself lucky.
The main thing you miss out on with ant is dependencies. Not only does Leiningen make it easy to depend upon other libraries, it also makes it easy to publish your library so others can depend on it. It's an ecosystem thing.
If that's not a big deal for you then the only advantage Leiningen could offer would be having one fewer violation of Greenspun's Tenth law in your life.
What put me off leiningen a while a go is the mention of Maven. I'm not going to touch anything related to Maven because at some point something will go wrong and I will be thrown into Maven hell. Maven is evil. It's the EJB of build management.
Also, I think that build management and generally dependency/package management is done completely wrong. The principle seems to be to manage large numbers of fine-grained, versioned, recursive dependencies. I don't think this will ever work well. The inherent complexity of that idea is always going to leak. We need to question the idea that modularity is always good.
I think you're probably right in the end, but a) what we've got is better than nothing (although it might be a big old dead end), and b) it doesn't look like there's a better option for building on other peoples' work.
I like two things about Leiningen. One, it provides a spectacularly simple hook into SLIME. Before "lein swank", the Clojure+Emacs+SLIME experience left much to be desired. Two, Leiningen handles dependency management, including transitive dependencies, quite well.
If you don't need or care for either of those things, you can probably stick with Ant.
In addition, leiningen has a nice plugin system to allow others to augment it. I found one last week which allows you to build clojure.org-esqe API docs [1]. I don't use ant often though, so it may well have similar capabilities.
In general, I imagine leiningen as a combo(-ish) between Ruby's rake and gem.