The less time there is to debate it the more likely people are to try it without having first been prejudiced for or against it by debate/news/rhetoric. There is a certain logic behind that, although I'd prefer that Bioware take the time to justify and communicate decisions to the community rather than simply hope the community react favorably after the fact.
All of that is certainly true.
It makes a certain kind of logic. While in general I'm relatively relaxed about any companies marketing plan for any of their products that I may choose to take an interest in, marketing does not exist in a moral/ethical vacuum. If there is information that the company knows will affect some consumers purchase decision (positively or negatively - but particularly negatively) then not revealing that information in a timely manner (i.e. prior to the product becoming available) is a highly deceptive practice.
I absolutely get that, if MP is included, Bioware might want people to have the opportunity to experience it's quality without exposure to prejudice, but if withholding the information causes people to purchase their product that otherwise (given more complete information) would not have purchased (or in my case, not have pre-ordered and waited to see) then I think we're tip-toeing into a very shaky legal area.
With regards to the second part of your post, I too, would strongly prefer they just say one way or the other. For the last few weeks, we've been told by various posters to the forums that, if it's in, Gamescom is the place we'll hear about it. We haven't heard about it, yet, but it still won't die (the conversation about it, I mean).
Surely it would be more beneficial to the 'hype train' for people to only be talking about what is confirmed and leave what is confirmed to be out alone. Because what is confirmed to be in is absolutely fantastic. Yet a casual visitor to these forums might be forgiven for thinking that DA:I will rise or fall based on something that not only isn't confirmed, it hasn't even been spoken about by the developers.