In the 6 seasons he qualified for the ERA title while Toronto, he was outside the top 25 in the AL in run support once. Regardless of the state of the Jays' offense, they pretty consistently put up runs for Halladay.
There's situational pitching, sure, but the notion that Halladay becomes a noticably worse pitcher when he gets a lot of runs I just don't buy. He gets his share of strikeouts because he has great stuff, but he's not a guy who is ever nibbling for the K, unless the situation absolutely calls for it. He always pounds the strike zone and is looking for the quick outs as opposed to the long at bat. He just has such great command and so much movement that he stays out of the middle of the plate and rarely gives hitters a pitch they really like.