Yes, I think many brands confuse transparency with not being professional. They are to very different things. Being human is what leads to confidence in who you are, what your brand represents etc.
I also believe being humble is key and have experienced first hand with our own brand and after selling our first agency, being very humble as we built the next has helped us grow tremendously. When you share along the way it is then your community who also helps build you.
Yes, good point about having a brand guide with tone etc. It shouldn't just be for the social team. For a brand to succeed they must know who they are from the inside out. We work with our clients on all of these things helping them further refine their brand, developing online brand personas, taking their offline brand online etc. For brands who have let their "brand go" a bit they are almost starting at square one again and are going back to rethink who and what they are. It can deliver such great results if they put the right focused effort into it.