actually you can hit multiple monsters with 1 arrow with pierce. you cannot hit the same monster with multiple arrows. yes, the arrows can be very tightly packed based on how far away the target is from you, but no, it's not very feasible.
imho, the main str of ms is that the way it works makes up for a low ar. if several arrows pass through the same monster, the first will hit or miss. if it misses, the 2nd arrow has a chance, then the 3rd, etc. so, with enough arrows, your ar can be effectively 100%.
however, what is the optimum number of arrows to take advantage of this effect? obviously, more is better, but there is a point where it's too many. in my experience, a 12 arrow spread gives me nearly 100% cth (or so close that i can't tell) which is why i suggest 12 after skills, or around 5 base points.
but, imho, if you make an ms zon (or an XX anything), you should max that skill. if ms is merely for support, 1-5 is fine.