I goofed on the second part of your question. I thought you were asking about limits.
What good is it?
The property damage portion of it really only protects your deductible, since your collision coverage is going to pay to fix your vehicle. I guess in the context of this thread though, it would also pay for an expensive rental, even if you don't have the rental reimbursement rider. There's more to it though, because your collision policy only covers "your covered auto". I'm going out on a limb here, but I'm guessing you've done a ton of mods to your vehicles and never reported the mods to your agent. Why report the mods, right? They'll just jack up your rates. Well, they can definitely try to get out of paying for those mods when you file a claim, because you never reported those upgrades and only paid a premium for a stock vehicle. If your vehicle is totaled, you'll have to fight every one of those mods when you're asserting a higher value for your vehicle, and they'll probably deny them because you didn't report them.
Also, you could have a bunch of other stuff in your vehicle that isn't covered by your collision policy. Say someone totaled your truck and the bed was full or tools. Maybe your truck burst into flames and burned up your laptop and your irreplaceable collection of vintage 8-tracks. None of that is covered by your collision policy, but you could make a strong case to cover it under your uninsured motorist property damage policy.
The uninsured bodily injury is way more valuable though. You could really be sunk without it.
Years ago my family was driving through town looking at Christmas lights and another car had just done a drive-by shooting at a nearby party and they t-boned us as they fled the scene. My wife's head went through the window and she needed a zillion or so stitches in her forehead. There's no way to put a price tag on that, but the price would have been zero if we didn't have the uninsured coverage.
Ask your agent about your uninsured coverage and see what he/she has to say about it. A good agent will talk you out of buying unnecessary coverage. Also think about asking your agent about any hypothetical mods on your vehicles (lifts, special tires, sound equipment, fifth wheel hitch, etc.).
