If I had to pick one kite, I'd pick 9m and it would cover me from 9 to 30 kts easy (I'd still need two boards: TT and HF)
But every kite/size has a sweet spot, which is just 3-4kts of wind range and one does not need to be a pro to appreciate it.
I am pretty happy with 2 boards and 4 kites 7-9-12-17 but occasionally there are days when I wish I also had 8 and 10 in my quiver..
That said, I never spend more than 2 seconds to decide which size to rig, unless I am not very eager to kite right away..
And going back to the OP, some guys start "what size to rig" routine, not because they are unsure, but because they want to socialize more than to kite