A lot depends on what you mean by better?
Is it being able to tap into the optimum power bands?
Is it for fuel economy?
If you want faster acceleration, you can make a close ratio gearbox
that is optimized for acceleration. The problem with close ratios
is that you often end up with very low top speeds.
Adding a sixth gear like from the Evo 7 to 8 gives you with a
slightly higher top speed while retaining the close ratios. This
means you can use a lower rpm when cruising and help improve
FC.
If you use five gear ratios but want to optimize for FC, then the
gears become much taller which may not be optimized for the power
band rpms for the car.
The WRC example quoted is a slightly different case, since there is
hardly ever any cruising done in that car. We are talking about
straights of at most a few hundred meters.
Of course, the problem with more gears to select from COULD mean
more work in shifting through the gears, but to be honest, I don't
find six gears all that daunting.
If it is any indication, the bigger cc Mercs and BMWs are all
running wif 7 speed gearboxes, I believe. 
