Just found this topic and I found it rather amusing.
I freaking hope the software development process isn't handled the same irresponsible way, without taking more than a handful of viewpoints into consideration. 
I'm sure your viewpoint was taken into consideration. However, it is but one of many opinions, and among those expressed in this topic, is the only one loudly against this measure. Surely you can understand that 1 < Many.
All you succeed in doing by adding that to the PM window is clutter/fuglify it up and and annoy those of us who don't want to look at a GUI with content that doesn't apply to the activity at hand.
One of this site's primary functions is a support site for SMF software. It may be a small inconvenience to some who use this site as a typical community. But 90% of the people here are here for support, and this message pertains directly to them. And considering that many who give support dislike receiving unsolicited PMs, then it's reasonable to assume that this would be useful.
What it boils down to is how it helps the SMF community. If it bothers you so much, you can make a topic about how easy it is to shut if off and link to it in your sig. But if this helps cut down on the PM spam that is unsolicited support requests, then it's a good thing. And if that helps one person who actually searches, or one Support Specialist to
not get a headache today, then it's a good thing.
Sorry if your real world experience hates this and feels strongly against it. But practicality often rules where logic would fail. If we're going to be logical here, then you wouldn't be wasting energy on arguing against this, it wouldn't be logical to do so, since there is an obvious benefit that outweighs one person's vehement arguments.