I'm not really one to question other people's experiences. Keep in mind that it's the Internet... and... well... "It's the internet". Anything can happen. It could be several people with the same unit who are accessing a public Wi-Fi signal, such as an apartment complex who was nice enough to offer it for free - I mean, the variables are numerous on why they would conflict without knowing WHY they are conflicting. I use an open network in my testing, and have never run into that, prototype, pre-release Beta-B, or post release consumer version of the product.
Assuming you have your own network (router), I can't imagine this being an issue.
In pretty much every case I've ever seen or had the ability to interact with, someone else's email address only applied when the unit was a customer return. So let's say I had the TV or Blu-Ray player, signed up to the services, and then decided to return it, or upgrade or whatever. The next person who got my unit would likely have access to my movies if the television wasn't reset properly. The same with units that were store displays, and an overeager retail associate plugged in his account so he could rock out to "Pandora at work". In those cases, re-assigning the device to a new account is pretty simple, and I've not seen where someone wasn't able to do it for that reason.
Netflix generally gives you a number of devices that you can connect. With my test units, I use my own account here, and have never run into an issue where some stranger's account information appeared on my unit. I've had to re-assign my devices on occasion because I broke the 6-unit maximum threshold for Netflix (I think in testing, we assign everything to my poor Netflix account, which changes devices every week - I'm sure any Netflix account auditor must be wondering what the heck I'm doing over here!), but connecting it back with the service is relatively easy.
I'm not led to believe that this would be a pain point in the ownership of the C6500. I'll stand back and see if anyone posts anything, but at work or home, it's pretty straightforward, and as noted above, shouldn't conflict with your home network.
Does that help explain a little? If not, post your concerns and I'll do my best to address them.