Well, if I can't tell, then I can be tricked. But if I'm fooled into thinking a computer is a human and I discover that trick later, I will be angry with and avoid the entity that tricked me.
> if you already know that it's a robot, are you still be willing to talk to it?
In the absence of some sort of really compelling reason to, no I wouldn't. It would signal to me that the company doesn't actually value me enough to treat me as a person. Also, I'm not interested in having a "conversation" with a machine, but that's more of a personal preference sort of deal.