One could probably fly a thousand times without prop guards and never have a problem, especially if all flying is in an open area. I flew mine as soon as I got it while waiting for the guards (ordered later) to arrive - nary a problem. After I got the guards, I put them on, but without the string. Flying close to the house (weather too crappy to make a trip to a better flying site), a gust pushed the Phantom up against the house ... it just bounced off and kept flying. A few days later, still tooling around the house I misjudged my closing distance to a residential light pole - guard hit the pole and the Phantom bounced away, but before I could react it attacked the pole a second time dead center between the guards - one blade severely cracked, a second chipped. It dropped six feet straight to the grassy ground. No damage to the Phantom but two blades trashed. Lesson learned. I now have the string installed. Keeping an eye on the string condition is a good tip.
If you're always going to be flying out in the open (no trees, poles, buildings, obstacles of any kind); prop guards are probably pointless other than perhaps making the Phantom easier to see. But if you occasionally get the urge to just get in a quick flight around the back yard, they may well save you some blades, if not worse damage.
As others have mentioned, landings gone slightly awry can tip your craft and possibly damage props. Additionally, if you like to try very slow lift offs (probably not a great idea) and a couple of props generate lift just a bit before the others, it can also tip the craft.
If you're always going to be flying out in the open (no trees, poles, buildings, obstacles of any kind); prop guards are probably pointless other than perhaps making the Phantom easier to see. But if you occasionally get the urge to just get in a quick flight around the back yard, they may well save you some blades, if not worse damage.
As others have mentioned, landings gone slightly awry can tip your craft and possibly damage props. Additionally, if you like to try very slow lift offs (probably not a great idea) and a couple of props generate lift just a bit before the others, it can also tip the craft.