Nigel Griffiths (of IBM Power Systems Advanced Technology Support, Europe - but he is also known as Mr Nmon) gave a great presentation on Shared Storage Pools at Copenhagen. This was also presented in the excellent Webinar Series on Power Systems Virtualisation.
Here's a small extract on why you would use Shared Storage Pools:
In smaller companies, there is no SAN team. The server guys are the SAN guys. The idea of Shared Storage Pools is to allocated LUNs to the VIOS(s) and a single VIOS command can allocate space to a Virtual Server (an LPAR). You can run a single command using cfgassist (VIOS's smitty) to allocate disk space. You can even use the Virtual Storage Management feature in the HMC GUI.
You can replay Nigel's webninar presentation in WMV format and download the presentation materials.
In the two years since this post was first written, there have been a number of other presentations on Shared Storage Pools. You can get hold of these from the Webinar series linked to in this post. (Links have been updated).