My reasoning for horizontal ascents are mainly for neutral bouyancy. When you are neutrally bouyant at depth, you are in a horizontal swimming position. If you were doing a dive where your ascent was along a slope to the surface; you most likely would spend the whole ascent following the contour of the bottom and remaining horizontal the whole time.
This for one is a reason to remain horizontal. Why go vertical to "ascend", when you can just remain horizontal. You can continue to watch the bottom in areas of good visibility, and you can keep your eye on the reference line on your ascent as well. By remaining neutrally bouyant, you can easily roll to reference the surface without any adverse effects on your depth.
Lastly, and most importantly... if something should happen that distracts you from your ascent [an OOA buddy, a lost mask, etc...] you are in a neutral bouyancy situation, and don't have to worry about loosing concentration and sinking or rising while you deal with the situation at hand.
e.g. when doing horizontal neutrally bouyant ascents, you can stop at any point and deal with a situation without having to concentrate on remaining at a constant depth when either positively or negatively bouyant.
While I am sure there are folks that ascend vertically while remaining neutrally bouyant, It has been my experience that the typical vertical ascent practiioner use their fins to ascend while being slightly negative, and not using breath control to ascend.