When Canon released the 24-70 II they did state that it is IS less in order to have uncompromised optical quality, and it does indeed have uncompromised optical quality. If you take Canon at their word rather than being sceptical there may be practical truth in it. The new 24-70 IS is very good; nearly as good as the 24-70 II and better than the 24-105 ( I have both ).
I would love to take Canon at their word if not for their incremental upgrade history, and examples of what is already out:
17-55mm f/2.8 IS - great lens, though not as good as the below lenses.
24-70mm f/4 IS - great lens, just about as sharp as 24-70 II.
70-200mm f/2.8 IS II - ultra sharp w/ IS, sharper than any lens in this class released ever.
100mm f/2.8 Macro IS - incredibly razor sharp, yet it also has the most complex IS system Canon makes.
I dunno, looking at those examples collectively I find it logically hard to believe that Canon just can't make an optically superior 24-70 f/2.8 IS happen, especially when the patents for the 24-70 f/2.8 IS already exist and were made public in July 2012
. IMO, it's just a matter of 'when,' not 'if.'
Granted, that is the big question and we may be in for a wait, but when it does happen the 24-70 II is going to drop in value greatly.
The original question was once you are the owner of a 24-70 II will you use the 24-105 anymore ? In theory you might say keep the IS lens for when you need it, but in practice you'll probably never have it with you when you need it.
If financials are not a concern I think both are cool to have because of the latter's better reach and IS; stick the 24-105 on a secondary crop body and you have quite a nice compact telephoto zoom. But, it is true that it will likely sit in the bag unused most of the time, as you always want to bring your best and generally do not want to lug around stuff you don't need - plus realistically you could get $700 for the 24-105.
On the other hand, if you only have the 24-105 it might be worth hanging onto to see what 2014 lenses bring re: 24-70 especially with the 24-105's low resale.