Restarting of management agents will stop/terminate the current running process in the host (Cloning, Image backup (VCB), vMotion, SVMotion) the vms will not be impacted but there is something called APD (All path dead) introduced in 4.0 i guess, if your ESXi host LUN/LUNs are in APD state there are more possibility that the hostd agent will not restart successfully. You will have to end up in restarting the host to remove APD condition.
So before restarting the managament agents make sure you do not have LUNs in APD condition. You can have a look at the below article
VMware KB: ESXi/ESX hosts in APD may appear Not Responding in vCenter Server