Host Named Site Collection (HNSC)

Host Named Site Collection (HNSC) has few benefit at the same time lot of trouble?

I have few question for the experts who is really associated with planning phase for global top level enterprise.

1. Do any global top level enterprise (e.g. having user base of more that 0.1 million) use it frequently?
2. In that case how many site collection practically they can keep under a web application? I know limit is 100,000. But in reality how much?
3. AAM is not possible in HNSC, then how do people handle that situation?
4. I know Microsoft himself using HNSC for hosting, but practically for which kind (Zone) of application companies are using HNSC.
5. Is it freequetly used for SharePoint 2010 on-premise application also?

Please share your thought on this.

Thanks in advance.