There is so much to learn when starting this hobby that I decided using a tray would help ensure that my hands were always held in the same position, thus adding stability and consistency. I also think that using a thumb and finger together (pinch) is a natural movement made comfortable by having opposable thumbs, that is reinforced by many activities over your life..like using a pencil, holding a fork or knife, etc. I can't think of many fine motor activities that use only the tip of a thumb. So I was better able to make small control surface movements using the pinch approach.

Of course it is possible to learn either way with equal success...but using the pinch was an already learned and reinforced skill and gave me one less new thing to deal with.