You can look into the past and find examples of the C's coming out on top without HCA or losing it during the series and still coming out on top. But to simply expect that is simply too much. Let's just put it this way, you would expect the C's to win against all the top teams with HCA right? An impartial party would expect other top tier (read non-Celtics teams) to also win their series with HCA.
Not saying it's mission impossible; but it's a whole lot tougher than you'd want. Also I don't have the exact statistic, but the team with HCA in the Finals has won a very high percentage of the series played.
The basics is that the best teams usually have the better records, too. This year, it's a bit more complicated. Both in the east and the west the teams are very close, and even a small stretch of bad basketball can lead to a lower seed, that doesn't show the team's real strength. The Spurs and the Bulls are simply the teams with the shortest bad stretches this season.
My opinion is that it's not enough to be the best team, and besides "luck with the refs"(=HCA), you need versatility in matchups. The Celtics have the pieces, but so do the Lakers, Spurs, Bulls, Thunder, Nuggets and the 76ers.
The Knicks, Mavs, Magic and Heat are dangerous, too. But to me these teams will win or die by their own strengths and weaknesses, and they are more likely to end their run in a bad matchup.