NFS will block if you run everything through one mount point.
I don't understand this, but then there's so much I don't understand about NFS.
This is a client-side architecture dependent issue, but for many unix systems, nfs requests for each individual mount point are serialised. This can create a per-mount point bottleneck, so the obvious way around the problem is to use multiple mount points, which can parallelize things to a degree.
Nick