Whether or not Harper wants to prorogue Parliament is irrelevant to the calendar. Parliament was set to re-convene Sept 17, which begs the question- why do we have to wait a full month for the throne speech? The main reasoning, or spin, proposed by the government centers around the Ontario election.
The first excuse- the government doesn't want to sit during the election, for fear the opposition will use the opportunity to score points for their perspective provincial counterparts. That logic is decidedly weak, particularly when the government controls the agenda, to some extent, and therefore has the means to assist the provincial PC's. A manufacturing bailout, more money for urban infastructure, etc, etc... a myriad of options. If anyone thinks Harper is timid about flexing his muscles in a provincial election, I suggest a re-read of the Quebec election. The opposition can play games, but I don't buy the theory that Harper wants parliament on the sidelines until after the vote.
The other excuse, making the rounds- the federal Conservatives want to allow Ontario party workers full focus on the provincial election. This reasoning is mostly absurd, because it follows that Harper is effectively allowing the opposition the same opportunity, which nullifies any advantage.
Beyond the spinning, the date of October 16th is relevant, because it ensures that we can't have dual election campaigns, and in my view that is the rub with the calendar. If parliament were to return as scheduled, Harper prorogue's and a new session begins immediately, the government could be defeated and an election is called. That is a potential nightmare scenario for Harper, given that Ontario is key for his prospects, and a McGuinty victory could influence a looming federal vote. I believe the Conservative braintrust has already concluded an election is likely after the throne speech, and is really clearing the calendar to ensure no conflict. That seems a more likely thought process, than the two lame excuses provided to date.