The key thing to remember here is that we're asking this very much in the middle of the interview, and I just want to see that they have some sort of system in place for remembering these things.
In my experience, most of the time, simply going "sure, I'll do that" and depending on memory to actually get it done is a negative signal. Maybe they are particularly gifted in terms of memory, but in my experience that is rarely the case.
In hiring we're willing to accept false-negatives, but never false-positives. You are correct, I may let a perfectly good candidate walk, but I'm willing to accept that as the cost of hiring a bad one is just too high.
IMHO the real problem is the strict barrier between people who are part of company and non-company. Imagine if we had a more permeable membrane with different departments hiring firing as they see fit (a faster flow than this structured hiring). This would test the candidates on working in the real environment rather than the whole magic and mysticism of made-up work scenarios in interviews.
In fact I wish we had a more libertarian system inside companies with more free markets rather than all the central planning which happens now
Unless your request was very specific and strange ("send me the completed k3-45 and t9807 forms"), in which case I'd have to jot /that/ down.
Should you not rather evaluate whether your request was fulfilled?
Again I do want to stress: This is a small detail as part of a larger interview. Screwing this one thing up isn't going to sink you, but we combine it with a bunch of other signals and observations to make decisions. I've been doing this for several years, and I'm very happy with the results.
Because it's just one of many possible courses of action for staying organized. For all you know the guy added it to his to-do list on his way to the car, or maybe he employs some other mechanic. While you could argue that an interview is a showboating environment where one is expected to signal certain desireable traits, I personally resent that these attributes seem to manifest in recruiters as specific actions that reinforce their perceptions of a good candidate -- if someone is unconventional then how is looking for specific signals going to tell you anything useful about them?