Is it unreasonable to add a person on linkedln that interviewed you? I want to add a person who did a phone interview with me. Is this seen as a bad gesture?
It's very normal and very common.
It's become an acceptable and public way to check someone out in more detail and vica-versa.
It communicates a "I'm still quite interested in the job after the call".
It's frequently seen as providing immediate references (including actual recommendations on linkedIn) and many folks can create resume's there too.
I think you want to play it by ear, and take refusal with a grain of salt.
Some companies advise against such links to insulate hiring managers from the expectation of followup after the interview. Others don't care or may even encourage the link.
If you had a good rapport with the person, if they stuck out to you as particularly worth keeping contact with - then give it a shot, but don't be upset if you get ignored. Conversely, I'd advise against doing it just because you had an interview - because it's pretty likely that in 3 years you won't have any idea who these people were, or why they mattered - and LinkedIn is really geared towards sustaining professional connections...
If you'd prune your connection tree of this guy if you don't get the job, I'd say don't invite in the first place. If you think that 3 months down the line, you'd want to read a status update from him - then go for it.
As an interviewer I would not be a fan of this.
My feeling is that LinkedIn is a way to form a network of trusted colleagues. After one interview I am not really a colleague of yours and certainly haven't formed a trust relationship.
It would seem to me a bit awkward. I would not hold it against you in terms of the interview process, but I would not accept the invite.