I could be way off, but like you mentioned Andrea_r, from what I understand a justification is that when an inexperienced WordPress user looks for a plugin for particular Twitter functionality, they see Twitter Tools, TweetMeme, Twitter for WordPress, Twitter This, etc etc etc and could certainly be benefited should there be an Automattic TwitterPress plugin that is unlikely to be developed by someone who doesn't know what they're doing, abandoned, etc.
If the only cause for concern is that GravityForms may lose sales or go out of business when there's and official FormPress, then I would be fine with that. (No offense to GravityForms, I use and love the plugin.) The problem I see is the problem that exists whenever the governing body (Automattic in this case) socializes a particular industry. When the official TwitterPress comes out, we'll risk losing the innovation that goes into Tweetmeme, Twitter Tools, and the others. When FormPress comes out, we'll risk Cforms II and Contact Form 7 being dropped, when Oliver and Takayuki decide their contributions just aren't worth the hard work anymore. And yes, we'll risk Carl no longer being motivated to make a comprehensive and well-written GravityForms, if the canonized FormPress makes it so that people will tend to not use it.
For what shall it profit a man, if he shall gain the whole world, and lose his own soul?