There have been cases where this has had a negative impact. I've even heard about an IO writing in the notes that she didn't believe the relationship was genuine, for several reasons, but listing the fact that the husband (PA) was walking a little ahead of the wife (Sponsor) and according to the IO, this was a sign that their relationship wasn't real. And this was just from what the IO saw in a very brief moment as they were leaving the embassy's offices. That being said, there is no real need for the sponsor to be there. And if the immigration officer wants to talk to the sponsor, they would tell the PA in the letter requesting for the interview that the sponsor should be available for a phone call during the time the interview takes place. Then, they may or may not call the sponsor.