Let’s be honest, nobody has made the full swap to the vSphere Web Client yet. I’ve been pushing my colleagues and some customers to start using the Web Client, but it just isn’t up to par with the Windows Client. And I hate to admit but when a host or VM is down and I have to quickly walk through my vSphere environment to check for active storage paths, check VM and host settings, I still use the Windows Client for this.
On the VMware Community there is a thread running that burns down the vSphere Web Client very badly, which in my opinion is not necessary. In other occassions VMware has already shown their listening and improvements to the Web Client are in the works.
Now with vSphere 6 there will be major improvements but instead of completely getting rid of the Windows C# Client, VMware has made some adjustments to the Windows C# Client as well, just to show they are listening and do feel your pain.
The Windows C# Client in vSphere 6 will be able to edit VMs of all hardware levels but can only edit v8 and lower features. In other words, using the Windows C# Client on a VM with HW Level 11, you can still change RAM and CPU settings (all v8 and below features) but you can’t change vSphere Flash Read Cache settings since this is a v9 and up feature.
Source: vSphere 6 – Clarifying the misinformation. Also see the comments section.