I value my family time, and I fully expect my team to value theirs. If I send an out of hours email I always preface it with the words 'FOR TOMORROW' and never expect a response. My team however is motivated and often putting in the extra hours to get ahead and stay ahead of our competition, as is summarised by Mireille below:
If you create a culture of shared belief, that one and all can make a difference and you treat your staff with respect and empower them to manage their own timelines then yes send an email at 10pm.
For it will be received with the intent in which it was sent - i.e "I know my boss has just seen/thought of/remembered/needs something and he/she is sending this to me before they forget again - I'll do this tomorrow (in work time) as I know they don't expect me to do it (or even read it) now"
We live in a global economy. Business does not stop and lots of companies have employees, customers and partners scattered across multiple time zones. And what about flexible working that most employers agree has various benefits; some people do their best work outside of traditional business hours. Do you really want to curb their success?