Seems like the people who wouldn't consider a boat with a keel guard either don't own a boat, or haven't ever priced out just how expensive fiberglass repairs can cost. IMO, those wouldn't be the type of buyer I'd want to entertain in the first place, as they've likely got some other ideas that may not jive with the actual experience of boat ownership, and would want a deal for something that's actually a benefit, and useful for mitigating possible major damage for pennies on the dollar.
To give you my actual experience, as well as my father-in-law's - I WISH I had a keel guard before I came across a submerged rock pile - thankfully it was at about 5mph, or it could have been a lot worse. If I had a keel guard, I would have likely had to replace it, but with no damage to the boat itself. I also do not beach my boat, so your naysayers are wrong in their assumptions in my case. My father-in-law NEVER beached his boat. He added a keel guard because he would fish as soon as there was open water, often running into debris at the beginning of the season. Again, terribly presumptive that a keel guard means beaching it, or abusing the boat.
If the assumption that "abuse" is running the boat in water that has debris in it, then the boat would never be taken on the water, as there's ALWAYS something in the water, it's just a matter of whether it's near you, or even visible. If all a person does is take their boat to the sandbar, and back to the launch, I guess that could color a person's view like that, but again, I'd be happy to avoid dealing with such assumptions.
Did you buy the boat to actually use it, or is it gonna be a trailer queen? It sounds like you're planning to use and enjoy your boat. I'd suggest you call up a fiberglass place and ask them how much it'd cost to fix a 6" gash on the keel that's only damaged the gel coat, with no fiberglass showing. I suspect your next move after hearing that price estimate will be to install that keel guard you bought, ASAP, and thank your lucky stars that you'll be able to have a little more peace of mind while you enjoy your boat, without worrying about the resale value that'd cause you to second-guess how you want to use your boat, and possibly lessen the enjoyment of ownership.