There's two items to consider - focus points and focus point size. More focus points means the focus point size must be smaller.
Let's start with the number of focus points - the reason for this is focus tracking. See examples below:
The first image is a Olympus E-3 with 11 points. The second is the 1D Mk3 with 40 something points. See the density difference? If you are tracking a bird with one point on the E-3, and it moves into one of the empty spaces in the AF grid, your E-3 will stop tracking it. It might not even be able to start tracking again when the bird reaches the next AF point. This is partly the processing requirements (to recognize the same subject). The 1D Mk3 doesn't have as much "empty space". Thus - it will track better.
This is most applicable to tracking - users of single point AF won't notice the difference between these two (except for focus point size issues). Without a large number of focus points, Predictive AF is not going to be reliable.
As for AF point size - the k-5 is notorious for having "large" focus points. A large focus sensor can be problematic because the single sensor can "see" multiple objects and will select one of those objects to focus onto - for example, if you are taking picture of a bird who is somewhat far away, the focus sensor might focus on the bird.. or something it also sees in the foreground (branches) or background (branches). It doesn't know what YOU want to focus on, so it'll just guess. Smaller focus points means you have finer control.
Smaller focus points have it's own issues, because if you don't have a good density of them, you have to use focus and recompose. For example, in the E-3 image - if those points were small, then if I wanted to take a picture of someone's face where the eyes are not at the center of the frame, there is no focus point I could select to focus on the eyes. I would have to focus and recompose.
In summary - quantity of focus points matter most when dealing with tracking. When dealing with single point autofocus, quantity of focus points doesn't really matter. Focus point size must also be taken into consideration.