Well... It depends on how and why you use the intake fans. As Great Gig already said, one have to take note of the dynamics involved. First let me ask a question.
Which set up will move most air through the case? (in the hypothetical situation that no air enters/exits the case other than through the fans and the fans all are similar and set to the same speed)
1) 1 fan in, 1 out
2) 2 in, 1 out
3) 1 in, 2 out
The correct answer is that all three set ups move as much air through the case. Why? A fan is constructed to move X cfm and it will do this. Not less, not more. So airflow is restricted by the bottleneck, the direction with the lowest number of fans. Set up 2 and 3 would move more air if the single fan is disabled.
My opinion about this:
This would be true only if there are not additional holes in the case and it is completely hermetic (imagin the pressure in the fans if there are no additional holes or places for air to go out or in except fans themselves...). If there are holes, then air will go out or in through them. The basic principle is that the air that enters the case and the air that goes out is exactly the same (so that the pressure in the case is constant, it can not increase or decrease continuously, an equilibrium must be met). At least after a small estabilizing time.
So if you put for example 2 fans blowing in x cfm each, and 1 fan sucking out x cfm, the total air (forced) entering the case is 2x. Then the same quantity needs to go out. As there is another fan blowing out x cfm, the additional air quantity (another x cfm, because 2x-x=x) will go out through the holes.
Similar case for the situiation with 2 fans out and 1 in.
In this examples I have not considered the pressure that the fans have to support, if there is a lot of airflow impedance, then the cfm that fans move is obviously smaller than the rated one. That is why holes are important (for example the ones in the front of the case, in a negative pressure situation with fans at the back of the case blowing air out).
And as some have said in the thread, it makes sense to add a fan blowing air in, even if we already have 2 fans blowing out, because it helps to direct airflow (force the input point, so air goes through this fan instead of through other holes in the case).
PS. Sorry for my English... if there is some language mistake please correct me