vSphere vswap files are often left unused. If they're stored on a dedicated datastore and that datastore goes offline, your VMs will continue to run.  I'm not sure what will happen the vswap file is in use and the datastore goes offline.  The use of dedicate datastores can come up when using array-based replication with SRM.  You can avoid sending many GB of useless vswap files if you use a dedicated datastore.  Most host-based replication tools can be configured to not replicate the vswap file and save bandwidth so there's little value in using a dedicated datastore.  In addition, if you're not overcommitting memory and have tiered storage, you can choose to put the vswap files on cheap and deep storage and not pricey flash.