Question: Does it really perform better than a single partitioned installation?
Why?
Answer: Even if the performance increase is non-existent, there are several reasons to split up your installation:
- If one file system corrupts, the rest of your data is intact
- A process gone (i. e. writing excessive log files) only fills one partition
- You can mount your /home directory in several installations
etc.
Wikipedia has a great summary on the benefits of multiple partitions.