A few times in the past, people have posted threads like

"I'm going away for the weekend, goodbye till Monday! :shame: "

then a few days later, it's

"I'm back, I said I'd be away but now I return! Did you miss me? What happened while I was away?"

and it's all a little pointless.

But on the other hand, I do see your point. Quite often, well-known and well-respected members come back out of the cold, even after an absence lasting years, yet their return goes unheralded. I think it'd be good if "I'm back!" threads were allowed for those who've been gone a significant time, whose return will actually matter. However, there's no accurate way to distinguish exactly when such a thread is OK and when it's not... so I guess that "I'm back!" threads might stay off-limits unless some better system comes up...