Trust me when I say that I wouldn't purchase anything that I didn't like, but as an example I was very torn between the 816v and 836v Focal speakers. There was about a $1,000 price difference and I wanted another opinion on the difference in sound vs. the value. Having a 2nd set of ears gives me the opportunity to have him tell me what he hears and what he likes or dislikes about them. We've been friends, college neighbors and roommates for more than 10 years so we have a pretty good idea of what the other one is looking for in a purchase so he can give me good advice the majority of the time.
As for a placebo check, it's just nice to have someone else confirm an improvement and my wife doesn't care enough to give most changes an honest listen before getting bored. She heard a huge difference with the speaker upgrade, but claimed no difference with the receiver swap that was a significant improvement for me.
I don't just trust anyone to give me their opinion, but he's the exception. Between his EE knowledge and appreciation for all music, both live concerts and home audio, he is a great sounding board for my thinking.