Prop guards are just that, guards. Depending on the impact during a crash, the guards could possibly snap off on a strong impact. The guards are there for accidental tip overs. Instead of the props touching the ground, the prop guards will protect the props. I learned how to fly this bird without the prop guards, then I purchased them. I tried them on for the first time today and I honestly do not like them. The guards get too much on the way of the camera while recording. Very annoying. That's just my preference. Although they are light, the extra weight could possibly affect maneuvering a bit. While they are installed on the quad, it makes the birds look so massive and awesome though. But once you get the hang of flying this bird, in my opinion, you do not need them.
99% of all prop damage is due to pilot error because we try to move to fast and try some things that we're really not experienced to do (let's keep it real) All it took for me to slow things down is once you have to keep purchasing props (at $30.-$40. a pop) then you start to take the time to learn your Q and treat it with respect. I learned my $30 lesson and haven't broken a prop since. Please don't get offended but in my opinion prop guards are for inexperience pilots who take the risk flying around obstacles (trees, cars, people). If you find yourself continuously breaking props then I suggest investing in some carbon fiber props; a lot stronger, but they can also do more damage. I know that Yuneec props are flimsy and break easily but sometimes that's a good thing.
I live in Wyoming and no matter how carefully you land it takes a couple of seconds for the props to shut down. A gust of wind (common here) has tipped the Q up and caused a spinning prop to clip the ground or pavement and boom $7.50 down the drain. Prop guards can be worth the money where gusty winds are a problem
I use a set of Yuneec ones when I check gutters and roofs,, saved my bird several times, they are a pita to put on, they are held on with screws, but will not come off if bumped, worth the money for certain applications