Many will agree the best answer to whether you should present all storage to all hosts is, "it depends." Here are some ideas that may help you decide if it's right for your environment.
1. I work for a largely Cisco UC shop so there are lots of UC apps being deployed, sometimes alongside general purpose VMs. Some of these UC apps need a great deal of datastore space due to call recordings or upgrades or thick-provisioning "requirements" from the vendor for supportability. In these cases, sometimes it's best to dedicate some UC-only datastores to the UC-only hosts.
2. If you have service levels or vSphere clusters that are mapped to levels of service or capacity, you may be able to separate your datastore service levels to back only those clusters that support those levels. So for instance you may have some SSD-only or hybrid datastores that are at a Gold service level. Only Gold-level clusters may get to use those datastores. And on down the line. Maybe 10k SAS drives are only mapped to the Silver cluster and 7.2k high-capacity SATA datastores are mapped to your lowest service level.
3. Perhaps you have different ownership of storage. Maybe one business unit, say, sales, brings in all the money to the company and the Sales department purchased additional storage for its applications only. Then perhaps you can segregate Sales storage from others.
4. VDI or vButt might be more reasons to separate storage - you only want certain clusters to use certain datastores.
Beyond some of these initial ideas, there's nothing inherently wrong with showing all datastores to all clusters. Perhaps instead of asking, "why do you have every datastore mapped to every cluster?" you can ask, "why should you separate the datastores between clusters?"
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Mike Brown
VMware, Cisco Data Center, and NetApp dude
Consulting Engineer
Twitter: @VirtuallyMikeB
Blog: http://VirtuallyMikeBrown.com
LinkedIn: http://LinkedIn.com/in/michaelbbrown