Does it really perform better than a single partitioned installation?
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Even if the performance increase is non-existent, there are several reasons to split up your installation:
Wikipedia has a great summary on the benefits of multiple partitions.
Multiple partitions are helpful when you want to upgrade your distribution, change to another distribution etc, especially a separate "/home" partition. All your data resides on the /home partition, so its very easy to reinstall a distribution without losing any of your data.
More info here - http://www.control-escape.com/linux/lx-partition.html
I mainly use the multiple partition for a couple of reasons: