With my 5D MK III is an f/2.8 lens really needed???

[lucuias]

5D Mark III + 70-200 F/2.8 is a great combination my friend.Aside the awesome shallow depth of field,I do sometimes turn off the flash and shoot with only available light at high ISO.



Model: Canon EOS 5D Mark III
ISO: 8000
Exposure: 1/80 sec
Aperture: 2.8
Focal Length: 70mm

[Mt Spokane Photography]

In extreme low light, I switch to primes, because I find that f/2.8 does not quite make it for many shots.
I use my 35mmL, 50mm f/1.4, 85mm f/1.8, and 135mm f/2. 
I could use a f/2.8 lens at ISO 12800, but I prefer a lower ISO setting unlless I'm really in a bind.  As it is, a few shots will always end up at f/1.4 and ISO 12800 thats ok when its just a few.
I tried using my 24-105mmL for a few images last year, it was a exercise in fuzzy images shot at too low of a shutter speed.  My subjects move, so IS doesn't stop motion, and flash is banned in a theatre.
I'd recommend sticking to a minimum of f/2.8, but you know best how much light you have.
Here is one when I was trying out a D800 and the f/2.8 lens was as fast as I had.  Its ISO 6400 with a ton of nr, and losing a lot of detail.  Fortunately, the D800 had a lot of detail, so it wasn't too bad.

 
My 1D MK IV at ISO 12800 and f/1.8
 

 
 

[M.ST]

If you want to shoot in low light and stop cation you need a f/1.2, f/1.4, f/1.8, f/2.0 or f/2.8 lens.

I like the EF 135mm f/2L USM and the EF 200mm f/2L IS USM.

[MarkII]

Is there any merit to this line of thinking that f/4 lenses are good enough to use for dimly lit scnese using the 5d MK III?

I do not think that you can get a meaningful answer to this question since no one here really knows what you shoot, what style you like, your budget, and so on...

Re the 70-200 f2.8 vs f4, you should answer really the question yourself.  Since you have the 2.8, use it to take the photographs that you bought it for in the style that you want, and then look at the resulting pictures to see how many were at f4 or below and how many were shot at a speed that needed IS. Obviously you will need to shoot in a mode that leaves you to choose the aperture to do this.

Re the other lenses, again, you should know whether or not you want to buy a 16-35 zoom vs a 35mm prime (or both) simply from the kind of pictures and framing that you want to take.

If you are still at a stage where you are not sure what pictures you want to take and how to take them, I would suggest buying a set of zooms covering a wide range. If budget and weight are an issue, buy the cheapest/lightest options or buy second hand.  Spend some time (months or years...!) shooting.  Look at the results and ask what you would like to improve and what pictures you simply could not take. Use that to work out what equipment you need.

[sagittariansrock]

I think one of the benefits of FF is the shallow DoF you can get for an equivalent FoV (compared to smaller sensors). So f/2.8 is better than f/4.
However, it really depends on usage. I think I shall have to make a decision between keeping the 70-200 f/2.8 IS (or getting the MkII) vs getting a 70-200 f/4 IS AND the 135 f/2 when I go full frame.  Note that either way the cost will be similar, so it boils down to whether I want the convenience of a fast zoom compared to the versatility offered by the combination of a slower but more portable (and more likely to be carried) zoom and a faster prime for shallow DoF and larger aperture when needed.