It won't speed up file access much, unless the partition the /home is on a faster device. It does allow for separate buffering though, so I guess there would be a minor speed increase from that under the right conditions.
Mainly though, it makes your file system more rugged. Filling up the /home partition does not result in the main filesystem crashing or being unable to update. You can also re-install at any time, and by selecting the advanced mode (partitioner) in Ubiquity you can specify the /home partition without formatting it. This speeds up the installation and leaves your data intact, even if you wish to format the / partition at that time.
A separate /home can also make data retrieval easier in the case of a crash. If you suffer from a failed release upgrade for example, the /home partition will be untouched and you can easily recover by installing or re-commencing the upgrade without being too concerned about your data. On one of my machines /home is on a separate drive, physically insulating the data from problems on the first hard drive.
Having a separate /home partition does NOT negate the necessity to do proper backups of your important data however. You should never entrust your most important files to just one drive or partition.
There are no problems with having a separate /home, as long as the person administering the system has the ability to understand and set up subsequent releases correctly.
Since Ubuntu itself requires less than 4GB, efficiently sizing the / partition is the key to success. Generally 15-20GB is more than enough for the / partition no matter you want to do, however if you want to install large database programs (oracle) setup large servers, or doing anything else that the normal desktop user will not do, then you need to understand the requirements for the system you are trying to build before proceeding.
I like to have two / partitions. Subsequent releases can then be put on the next partition, allowing me a complete fallback to the previous release in the event of problems. This has stood me in good stead recently, when problems were encountered configuring shares in the LAN. The previous release could be booted while the problems were sorted out, minimising disruption for people using these shares. Two 15-20GB partitions do not take up much space on modern hard drives.