I understand what you say, I understand where and which message you want to transport. I fully agree that it is desirable to have such small helpers.
However, in this case it still is the wrong message. If you want to help users, the right message could be "... or select any face to place your sketch on that in case you fancy an unstable model. However it isn't possible now anyway, you would have to cancel this function to do do so".
Don't get me wrong: I don't say we should not listen to the newbies, but the newbies should listen to the long term users too.
I get the impression that you had a very well meant idea with this example in mind, because you had some learning experience and want to share it with other users.
It makes me feel uncomfortable seeing that you insist on your opinion without really reading what it's all about and make yourself familiar with the matter. Read about attachments which I still think would be the right thing to optimize.