I have no worries that this will just be a very short cameo. It's not going to affect the story much one way or the other.
If you like Smith (like me), you'll probably think it's cool. If you don't like Smith, well, it probably lasts less than a minute and the rest of the episode and Capaldi will stand or fall based on its own merits.
Worries about the "affirmation" aspect are just over blown.