I agree with the sharing. There's too many positive benefits from sharing that outweigh the negatives. I mean, yes you can try and rationalize with examples of one bad angler bonking a fish, but then you also have to realize that sharing information may help to create an an ambassador for the sport, who becomes an active and contributing member of the community. By helping one person, you may never know the positive ripple effect that you've created.
Also, it's so easy to forget that we've all received help along the way. I mean someone must have helped you by sharing. Even if it's the guy selling you the rod or a book or an article or word of mouth or a friend or youtube... someone was sharing information and you received it. Which reminds me.... I should thank rickr for his primer on nymphing. I'm still terrible at nymphing, but I credit rickr for getting me into my first fish on the Bow. I've never met rickr, but he took the time to share his nymphing info and I used it to catch a few fish. Hopefully as I get better and learn more, I'll be able to pass what I've learned to other people (not just to friends and family, but to strangers as well).
Cheers.