Benny,
Generally it's best to offer someone the opportunity to chat without being invasive to their browsing. Consider the mass rioting that happened when people learned that Facebook was using their personal information to target ads. Of course, as marketers we realize that ALL websites do this but the average visitor has no clue. Therefore, they find it alarming when they're automatically placed into a chat.
I believe the point the other chat providers are trying to make is that this should be triggered by a clearly visible button. For example, a red gradient that encourage users with questions to "Chat Now" instead of the smaller graphic on Facebook. It's also easily revised to a "We noticed you might need some help, would you like to chat with us?" sort of message.
You're absolutely right that usability and design plays a huge factor in the experience. And they're right that there has to be a balance of sales as well.
Ryan, as an agency we actually recommend ContactAtOnce because of their integration to CarSoup.com and Cars.com. We're assuming AutoTrader will follow suit sometime in the near future. Plus, the fact that they're integrated into the automotive vertical can be an advantage.
I was offering a personal recommendation on ActivaLive because our analytics showed much higher leads produced from a custom developed, integrated solution than a plugin. However, it ultimately gets down to the processes in the store and level of expertise available. It definitely took a LOT more work to get there. 
