Ahh, you mean "makes Catalyst's switching useless". I thought you were talking about making radeon Prime useless.
At least I understand what you're saying now, even if I still don't agree with it
Useless was probably the wrong word to use but personally i find it annoying to have to switch between cards "manually".
To give you another example imagine if you could get 100hp more out of your car but you had to stop, get outside, open the engine bay, press some buttons, and then be able to use it. Its not the most "user friendly" way to do things. I would perfectly understand it if there are technical reasons (ie X doesn't support fancy stuff like that) for it but still its not "perfect".
Using automotive analogies in the computer world has always been a high risk activity, but if you want to go there....
My Jeep has locking hubs - you get out and manually engage them if you want four wheel drive. Upside is better fuel economy on the road.
Modern V8's often have mechanisms to automatically disable 4 of the 8 cylinders under light load, but getting good fuel economy requires "reverse engineering the computer" in your head to figure out when it's safe to step gently on the gas. I would pay extra for a manual switch to lock it in 4-cylinder mode even if I had to pop the hood to switch it.
For what it's worth I don't think anyone is claiming the current multi-GPU support is perfect, but AFAIK it *is* better than anything else on the market right now.