In my opinion, putting your call on speakerphone is generally utterly useless. Why? Let me count the ways…

1. The reason you’re probably putting your call on speakerphone in the first place is because you want to be free to move around/do something else/look something up/play internet games/check Twitter/check Facebook/etc. A couple of things about this:

a) This needless multi-tasking will not help you remember what the person is calling about. If it’s business, all the more reason you should listen… especially if it’s your boss. If it’s personal, you might end up forgetting something important (mother-in-law’s birthday plans, commitment to drive someone to an appointment, an actual appointment for yourself, etc), and then have to face the consequences.

b) Also, there’s a good chance that the person on the other end will notice you’re not really paying attention to them. This can also lead to all manner of unpleasant confrontations and consequences.

2. It’s annoying to everyone else around you who is not part of the call. There’s this guy on my floor in this office building who seems to put EVERYTHING on speakerphone. I don’t know if that’s true or not, but every time I walk past his door, he’s on speakerphone, and I can hear it clearly in the hall. I can only imagine what the people in the offices on either side of him think. If I was one of them, I might’ve strangled that guy by now (or at least destroyed his phone).
Now, there is a case which I can think of in which the speakerphone might come in handy– that is, if you have multiple people listening in on the call, and it makes more sense for them to hear what the other person has to say all at the same time, then use of a speakerphone could possibly be justified. BUT notice that this scenario doesn’t include multitasking! This is a scenario in which people are actually listening to the caller! What a concept.