Leaves aren’t that hard to pull off with Tool>Geometry>Insert Mesh and Tool>SubTool>Append. Or with SubTool Master. I’d probably use the latter.
Append multiple leaves together as subtools, and use TransPose to quickly maneuver each into a good position. The idea is to create leaf clusters. Then user SubTool Master to merge all of them into a single subtool. You could now repeat if you wanted, creating clusters of clusters.
Now append those clusters onto your tree, which can be created via ZSpheres. Create three of four different clusters so that repetition doesn’t become noticeable – something that you can also protect against by rotating the cluster to different angles each time.
The simple truth is that you don’t need to have every leaf actually coming off of the branch in a natural way. Trees have thousands of branches, many of which are simply twigs. You’re not reasonably going to model all of them. So if the leaf clusters are set up in a natural way such that the leaves look like they would be attached to twigs, then nobody is going to notice the lack of those twigs. They’ll just see a mass of natural-looking leaves, and assume that the twigs are present.
If you’re working on a project that will actually zoom close enough to see twigs, well it’s unlikely that this will apply to the whole tree. Only flesh out that extra detail where it will actually be visible.
Heck, you could even use ZSpheres to quickly create twigs as you’re creating your clusters to begin with.
Bottom line: Always remember the #1 rule of digital art – if nobody is actually going to see a detail in your shot, then don’t bother creating it.