Having mobile presence or not?

Location-awareness of others, also expressed as a way to afford "mobile presence" has been supported by various interfaces. I did an tentative review in my phd dissertation with some categories of the types of interfaces and metaphors that are used to do so. The coming of applications such as Jaiku, Twitter (not to mention Dodgeball, Loopt or Wayn...) are examples of such. While I am interested in this and acknowledge that some systems are better and more interesting than others (Jaiku being my favorite for its very simple interface and the way location is captured/displayed), some people expressed criticisms over them. For instance, Janne Jalkanen says that:

There's just not enough benefit in telling everyone where I are and what I am doing so that I would actively use it. I don't mind the presence on the IM networks, because that's a necessity of those networks - you can't connect unless people are online - but mobile presence is useless, because everyone is online all the time anyway. And wasn't the whole point of cell phones that you would be no longer tied to a particular place or time or situation: you can call anyone anyplace anytime (barring some social conventions against calling people in the night)? However, I know there are tightly knit groups which love these kinds of applications, because they are living 24/7 closely anyway. But I am not sure I even want to know where my friends are. I think it would just make me bitter to know that they are out partying, traveling or otherwise enjoying themselves...

On his side, Niko Nyman have other concerns:

These services suffer from Metcalfe’s Law. They’re only useful if your contacts are using them as well. (...) the effort-to-benefit ratio is not good. I could see there being a benefit of knowing whether a friend of mine is available for interruption, or seeing where my friends are going on a Saturday night. But this assumes that a friend posts updates 24/7 about what he’s doing and where, yet I need that information for maybe one hour a week, possibly less. Realizing this, the 24/7 posting just seems like too much effort.

Then there’s that aspect of just keeping tabs on my friends’ updates “for fun and entertainment”. It could be just me, but watching what others are doing, described in three words, sounds like the world’s most boring reality show.

Why do I blog this? even though I don't necessarily agree with those comments (for example "mobile presence is useless, because everyone is online all the time anyway"), they are relevant viewpoints to discuss the importance of location-awareness.