A) It negates any issues with Microstuttering caused by CrossFire or SLI.
B) The Phenom II 965 is faster than the A8-3850 in Processing
C) The 6770 is a lot faster than the 6670 leaving you with a faster experience in games that don't scale well with Crossfire or SLI.
Basically it is always better to have one Strong GPU than two Mediocre GPU's if you have the choice at this point in time. But I also think the 6770 is likely to just plain be faster than the A8-3850 and 6670 even when in crossfire but I am not certain. Also consider the branding name is the 6690D2 which would imply that it is slower than the 6770 but I haven't seen any Crossfire benchmarks with the 6670 only the 6570.