There are a couple of reasons this might not give you the best performance.
1. If you cover Noopl's microphones, you make them less able to pick up sound (like putting your hands over your ears).
2. Take a look at this picture of a common situation for using Noopl — sitting at a table with others.
Noopl's microphones are designed to be most effective when picking up sound best from sources that are around it, more than simply above it. Imagine a line from each person's mouth to the iPhone (the Noopl is attached to the bottom of the iPhone). That's the "sweet spot" for Noopl's microphones. Noopl is less sensitive to the areas "above" where you'd drawn those lines.
With that in mind, if your iPhone was vertical and in your pocket, you've then changed the orientation of the Noopl's microphones. You'd likely hear yourself very loudly and not hear the sounds as well that are coming from in front of you.