First, to restate the obvious:
Attention from those interested and able to buy is worth more now than ever before. Companies like Google, Amazon, Daily Candy, Netflix, Target, and on and on traffic in attention. It's their primary asset. Individuals are also valued and respected in large measure by the quality of attention and trust they earn from their publics.
So, if that's so obvious, why are we so cavalier about it?
If someone stood in front of your office and lit $100 bills from your petty cash kitty on fire, you'd call the cops. But people at work waste the attention of their peers and your customers/prospects at the drop of a hat.
Every interaction comes with a cost. Not in cash money, but in something worth even more: the attention of the person you're interacting with. Waste it–with spam, with a worthless offer, with a lack of preparation, and yes, with nervous dissembling, then you are unlikely to get another chance.