The benefit of using proxies is that the geometry is stored in an external file, saving space from the main scene. But really, you can render thousands of instanced meshes (instances... not copies) getting similar render times. Futhermore, the proxies add a small penalty to the render time, i don't know sure why... probably because the geometry must be loaded from disk.
As instance, here are the results of a render time test, using 1500 trees (200k faces each one):
Forest with meshes: 1:06
Forest with proxies: 1:15
Instanced meshes: 1:27
Instanced proxies: 1:37
The difference is not spectacular, but as you can see, the times using meshes are slighty better. I have used Forest Tools (Utility panel), to convert the trees from Forest to instanced objects.