Apple certainly does to a nice job on UI. Still, I can't imagine how you can reason that not having more choice ("inferior" -- in your own words -- or not) can't be a good thing. You sound as though OSS just junks up the internet like litter along side the road or something. My main point is that even if you dont' like *any* of the OSS alternatives out there that just the sheer fact that there are alternatives to the status quo is a good thing because it forces Apple to adapt in ways it might not otherwise adapt, and (perfect interface or not) it shifts the power a little bit. (Slightly off topic, but you realize Darwin, the base of OS X, is OSS, right?)
As for the tabs with Chax -- I don't think tabs are an idea that you can "steal", at least not in the sense of the way we use them in our browsers, the way Adium uses them in chats, etc. Tabs are more of a UI feature that are generic enough that virtually any app could make use of them. i.e. I don't think comparing Apple incorporating tabs into iChat is comparable with Apple incorporating DashBoard into their latest OS.
In fact, I think that Apple's credibility depends on having apps like DashBoard ship with OS X, incorporating tabs into iChat and otherwise staying on top of their game -- not because of something specific about DashBoard or iChat, per se, but because they need to stay on par with what other apps offer in terms of features in an out-of-the-box configuration. Otherwise, they're going to be peddling outdated software that actually depends on (potentially unsupported) 3rd party components to be competitive.