I know this post wasn't meant for realtors, but in equal effort to help shed light on the situation from an agents side, I thought I'd contribute.
The effort to develop a competitive bidding environment is the best and most effective way in achieving a strong sale price for a property. That should always be a goal of an agent working in an ethically correct manner. It would be ethically wrong to aim for a quick sale, knowing there's more potential, unless the owners priority is a quick sale more so than the highest price possible.
The deadline dates are only a partially effective tactic. It definitely can put some pressure, as well as, protect a buyer who doesn't want a valid offer on the table longer than a few days. Most agents/owners see through this and know that the potential buyer will likely still be there after the expiration date, unless the property hasn't been getting a lot of action.
I believe I work in a very honest manner, with my seller's goals and interests steering all decisions I make. But with that said, a lot of agents are still selfishly driven. If you're in a position to do so, and comfortable with making strong offers (I understand this is probably not the typical new investor), then short contingency periods and letting the listing agent represent you works a lot. I'm not saying you should let agents double end, or that that is the best practice, but if the goal is to lock up properties faster and easier before a competitive bidding war happens, it does work. I would recommend educating yourself so your comfortable enough with the transaction process before you let an agent double end, and the seller has to be fine with this as well. My only experience is in Los Angeles, so that's where my views are shaped from. I know standards and customs change from city to city.
Cheers!