Why? Because it shows people as being online, when they're not. (And no, this isn't unique to my client, AFAIK all AIM clients work this way.) So now whether or not someone shows up in my blist is no longer a reliable indicator of whether or not they're online.
Worse yet, in multiprotocol clients (e.g. Trillian, Kopete, Pidgin, etc.) mobile clients are treated as online and active. So if the person is, for example, online but away through Yahoo, the client will still direct messages at AIM-mobile because it's not away.
In short, "online" no longer means "online". There is something fundamentally broken about that.
Fair warning: I'm going to stop checking for the little mobile icon. If I see you on my blist, and you're online (i.e. not away), I'll assume you're OK with being IMed. If you're not OK with this, then don't put your phone on AIM. Text messages work just fine, people. :P
[Edit: By "IMed", I mean, I expect you're OK with having a normal conversation, as if you were on AIM via a standard AIM client.]
Edit #2: To summarize/make things clear:
- I'm not going to treat mobile AIM any differently than normal AIM, period.
- If my client says you're online (mobile or not), I'll assume you're online and amenable to conversation.
- If you're online, I leave a message, and you consistently don't respond in a reasonable time, I'll think it rude (or at least inconsiderate).
- Barring a very compelling argument, I'm not open to reconsidering this. I believe the above points constitute good netiquette, so bitching at me about it will garner no sympathy. ;)