As a practice, I will never give a payment address via Skype (etc) that is not GPG signed by me. I also have this policy posted on my forum profile, and this has been displayed for a long time. This way when someone either hacks my Skype account or is able to impersonate me via Skype or otherwise, without a PGP signed message with a payment address, it will be the fault of whoever supposedly sent funds to the scammer.
I personally do not trust the security practices of Skype or most email providers as they are not designed to be methods that people do business when one person send money via a way that cannot be reversed
That's correct to give a warning. However, if an account is still being used by you and is under your possession I think whatever happens on it is still your responsibility. Unless the warning would be mandatory to read before receiving your Skype name, but that wouldn't be possible as there's no features to allow it (and it also opens the door to fake scam attempts). It wouldn't stop potential people who you are selling something to you from being scammed either. People who have had business contacts for awhile will get a false sense of security when talking to you and be more likely to just conduct a trade without going through the proper measures, more so with what your warning is stating. There's still many scams that could be pulled off without providing a PGP signed message with a payment address.
I also don't trust the security practices of most instant messaging services or email providers. I've seen countless businesses that conduct business through Skype or other apps that are similar have their accounts hacked and used to scam unsuspecting victims.
From my profile:
Skype:quick.seller I will never send you a payment address via Skype that is not PGP signed
It is not possible to see/get my Skype address without also seeing my disclosure about needing a PGP signed address.
If someone were to get a PGP signed address that was not signed by my PGP key (or one that does not verify) then it is their own fault for not checking. I have previously gotten payment addresses for a very large transaction whose PGP signature did not verify, and my response was that the person needed to resign the message before I would be willing to send funds to them.
Sure this could open up the possibility to fake scam attempts, however like you said the security for Skype (etc) is not the best and should not be relied upon, so instead of relying on their security, people should rely on the security of PGP.