1) Read your stem/fork instructions and they'll tell you to have 3mm - 5mm of spacer or steerer above the stem so there is room for the top cap screw to preload the bearings. You could do this with just the stem, but then you don't have as much clamping surface, which I dislike...
2) I personally don't like the idea of cutting a fork to the exact
size, because then there isn't much room to adjust fit, and transferring the fork to another bike or selling it can get difficult because its so short.
3) I like to stem mount my bike computer, and having just enough stem to stick up to the same height as the computer makes me feel that it's a bit more protected.
As a result, I tend to cut steerers to be 10-15mm longer than need be and put spacers up top. The result tends to be something like this
, which I've been quite happy with.