After refusing he actually changed his mind but aL was full so they didn't take him, but he actually felt like joining at a point fairly recently, maybe in preparation of TI3.
As for DK he's picked all the time tbh, I don't think he was ever a great lategame carry in a game I've seen, he did alright, did some decent sustained aoe damage and shit but he was never like "omg this guy is completely carrying his team" like you can see an AM, void or PL do in late/super late game. In fact I've seen plenty of very well farmed DK enter lategame and be completely worthless(partly due to item choices, he has a lot of viable items and you can definitely build completely wrong stuff), waste all the farm and sometimes lose the game because the farm was on DK instead of a more impactful hero.
The reliance on his ultimate, the lack of damage increase other than a fairly small splash, the lack of an effective defensive mechanism against burst damage, all of this imo makes him a very average hero. A good 2, a terrible 1, works well in multicore setups involving magnus or darkseer. That's how I see him at least and I don't really see myself changing opinion because I've seen DK being played for months/years and it's always been this way. He's never been bad, but he's never been great either.