@rdalcanto: We find the round display useful primarily for the following distance display in seconds. It helps us calibrate our following distance. You might not need it though.
Two other reasons to consider the display that I can think of. If none apply to you, it should be fine to forgo it.
1. You're paranoid and want to know that the system is functioning as designed. The display will let you know when it detects a pedestrian/cyclist/car in your lane ahead of you. It's nice to have, but not critical.
2. You want the easiest access to rarely used settings. The display has buttons on it that will let you cycle through the functions, adjust the sensitivity of them, the display brightness as well as the volume of the audible alert. (though my opinion is that the lowest audible setting is still WAY too loud for non-collision alerts, since we have a sleeping baby in the car sometimes). It might be possible to adjust the settings through the phone app, but we haven't tried it yet.
@araxara: speed limit indicator function isn't great in its current incarnation. Doesn't show in the little display, only via the phone app. So if your phone isn't mounted somewhere permanently visible while you're driving with bluetooth on and the MobileEye app running, you don't get the indicator. Used it a few times, and it works well, but remarkably inconvenient for me. Would rather Tesla build in a speed limit indicator either in the dash or the map display like some other cars do. Granted, the data might be off/old (the MobileEye seems to do image recognition, and therefore gets the current speed limit), but there are few roads I travel where I get on AFTER the speed limit sign and there isn't another for a very long way and therefore never get the indicator. If those types of road situations are common for your location or where you travel, it might not be that useful.