question about AF assist beam

Looking to buy an ST-E3-RT for my 600EX-RT but have a question about the AF assist beam.

Someone posted the following review on B&H:

For what it's worth, this transmitter does radio control off-camera speedlites... And that's all it does. Which is a bad thing. It no longer has the autofocus assist beam for focusing in low light conditions -- and low light conditions are possibly a principal reason you're using this equipment.

I'm trying to find out what the limitation really is. I'm still a newbie to using advanced features on speedlites. Up until recently I used to only shoot in E-TTL mode and made use of FEC as needed.

Could anyone provide a simple explanation and provide a practical example of what this limitation is? Did the ST-E2 unit have this as well?

Unfortunately to use AF assist, you must have and use an additional 600 EX-RT on camera. All of the features of the ST-E3 Transmitter are available albeit in a heavier package. However AF will work. On your master 600 EX - RT which is on camera, you have two options. Use the master as an active flash for fill if you like or have the master only transmit with AF Assist and not have the flash fire.

Limitation:

Ok, let's say you have a strobist-style photoshoot in a very dark area.
If you use your radio transimitter without AF assist beam, there is a chance you won't be able to autofocus the your lens.
A radio transmitter with AF assist beam insures autofocus (fast and accurate), no matter how dim the ambient lighting is.

I'd promise "assists" rather than "insures"...

apersson850 wrote in post #15515326
I'd promise "assists" rather than "insures"...

Well, yes "assists" is a better word.

Anthon wrote in post #15515259
Limitation:

Ok, let's say you have a strobist-style photoshoot in a very dark area.
If you use your radio transimitter without AF assist beam, there is a chance you won't be able to autofocus the your lens.
A radio transmitter with AF assist beam insures autofocus (fast and accurate), no matter how dim the ambient lighting is.

I think in that case where you are taking that much time to set up a shoot I would just use a modelling light or some other method. A small non too bright LED flashlight would work. Most cameras are pretty good in low light I would think. What is the percentage you shoot in those types of conditions?

I have two 600 flashes and the ST-E3. In a studio set up my 5D3 focuses well but have not experienced such a low light situation. The ST-E3 sits on it. Run and gun in low light the 600 sits on the camera. I will be ordering a 3rd 600 pretty soon.

I just picked up three of the 600's and the transmitter and was shooting at night outside with little to no ambient light. We resorted to using a flash light to focus with. I have a little laser pointer that will be less obtrusive at weddings, but pretty much a pain to try to fanangle the camera and laser pointer by yourself.